diff --git a/subprojects/gst-plugins-base/gst-libs/gst/pbutils/codec-utils.c b/subprojects/gst-plugins-base/gst-libs/gst/pbutils/codec-utils.c
index 797fc88096..add0ffb624 100644
--- a/subprojects/gst-plugins-base/gst-libs/gst/pbutils/codec-utils.c
+++ b/subprojects/gst-plugins-base/gst-libs/gst/pbutils/codec-utils.c
@@ -1876,7 +1876,7 @@ gst_codec_utils_opus_parse_caps (GstCaps * caps,
  *
  * Creates Opus caps from the given parameters.
  *
- * Returns: The #GstCaps, or %NULL if the parameters would lead to
+ * Returns: (transfer full) (nullable): The #GstCaps, or %NULL if the parameters would lead to
  * invalid Opus caps.
  *
  * Since: 1.8
@@ -2042,7 +2042,7 @@ _gst_caps_set_buffer_array (GstCaps * caps, const gchar * field,
  * Creates Opus caps from the given OpusHead @header and comment header
  * @comments.
  *
- * Returns: The #GstCaps.
+ * Returns: (transfer full) (nullable): The #GstCaps.
  *
  * Since: 1.8
  */
@@ -2102,7 +2102,7 @@ gst_codec_utils_opus_create_caps_from_header (GstBuffer * header,
  *
  * Creates OpusHead header from the given parameters.
  *
- * Returns: The #GstBuffer containing the OpusHead.
+ * Returns: (transfer full) (nullable): The #GstBuffer containing the OpusHead.
  *
  * Since: 1.8
  */
diff --git a/subprojects/gst-plugins-base/gst-libs/gst/pbutils/descriptions.c b/subprojects/gst-plugins-base/gst-libs/gst/pbutils/descriptions.c
index 2544aa9a98..e5f0275b82 100644
--- a/subprojects/gst-plugins-base/gst-libs/gst/pbutils/descriptions.c
+++ b/subprojects/gst-plugins-base/gst-libs/gst/pbutils/descriptions.c
@@ -925,13 +925,13 @@ caps_are_rtp_caps (const GstCaps * caps, const gchar * media, gchar ** format)
  *
  * Returns a localised string describing a source element handling the protocol
  * specified in @protocol, for use in error dialogs or other messages to be
- * seen by the user. Should never return NULL unless @protocol is invalid.
+ * seen by the user.
  *
  * This function is mainly for internal use, applications would typically
  * use gst_missing_plugin_message_get_description() to get a description of
  * a missing feature from a missing-plugin message.
  *
- * Returns: a newly-allocated description string, or NULL on error. Free
+ * Returns: a newly-allocated description string. Free
  *          string with g_free() when not needed any longer.
  */
 gchar *
@@ -975,13 +975,13 @@ gst_pb_utils_get_source_description (const gchar * protocol)
  *
  * Returns a localised string describing a sink element handling the protocol
  * specified in @protocol, for use in error dialogs or other messages to be
- * seen by the user. Should never return NULL unless @protocol is invalid.
+ * seen by the user.
  *
  * This function is mainly for internal use, applications would typically
  * use gst_missing_plugin_message_get_description() to get a description of
  * a missing feature from a missing-plugin message.
  *
- * Returns: a newly-allocated description string, or NULL on error. Free
+ * Returns: a newly-allocated description string. Free
  *          string with g_free() when not needed any longer.
  */
 gchar *
@@ -1010,13 +1010,12 @@ gst_pb_utils_get_sink_description (const gchar * protocol)
  *
  * Returns a localised string describing an decoder for the format specified
  * in @caps, for use in error dialogs or other messages to be seen by the user.
- * Should never return NULL unless @factory_name or @caps are invalid.
  *
  * This function is mainly for internal use, applications would typically
  * use gst_missing_plugin_message_get_description() to get a description of
  * a missing feature from a missing-plugin message.
  *
- * Returns: a newly-allocated description string, or NULL on error. Free
+ * Returns: a newly-allocated description string. Free
  *          string with g_free() when not needed any longer.
  */
 gchar *
@@ -1065,13 +1064,12 @@ gst_pb_utils_get_decoder_description (const GstCaps * caps)
  *
  * Returns a localised string describing an encoder for the format specified
  * in @caps, for use in error dialogs or other messages to be seen by the user.
- * Should never return NULL unless @factory_name or @caps are invalid.
  *
  * This function is mainly for internal use, applications would typically
  * use gst_missing_plugin_message_get_description() to get a description of
  * a missing feature from a missing-plugin message.
  *
- * Returns: a newly-allocated description string, or NULL on error. Free
+ * Returns: a newly-allocated description string. Free
  *          string with g_free() when not needed any longer.
  */
 gchar *
@@ -1116,14 +1114,13 @@ gst_pb_utils_get_encoder_description (const GstCaps * caps)
  * @factory_name: the name of the element, e.g. "giosrc"
  *
  * Returns a localised string describing the given element, for use in
- * error dialogs or other messages to be seen by the user. Should never
- * return NULL unless @factory_name is invalid.
+ * error dialogs or other messages to be seen by the user.
  *
  * This function is mainly for internal use, applications would typically
  * use gst_missing_plugin_message_get_description() to get a description of
  * a missing feature from a missing-plugin message.
  *
- * Returns: a newly-allocated description string, or NULL on error. Free
+ * Returns: a newly-allocated description string. Free
  *          string with g_free() when not needed any longer.
  */
 gchar *
@@ -1145,7 +1142,7 @@ gst_pb_utils_get_element_description (const gchar * factory_name)
 /**
  * gst_pb_utils_add_codec_description_to_tag_list:
  * @taglist: a #GstTagList
- * @codec_tag: (allow-none): a GStreamer codec tag such as #GST_TAG_AUDIO_CODEC,
+ * @codec_tag: (nullable): a GStreamer codec tag such as #GST_TAG_AUDIO_CODEC,
  *             #GST_TAG_VIDEO_CODEC or #GST_TAG_CODEC. If none is specified,
  *             the function will attempt to detect the appropriate category.
  * @caps: the (fixed) #GstCaps for which a codec tag should be added.
@@ -1204,7 +1201,7 @@ gst_pb_utils_add_codec_description_to_tag_list (GstTagList * taglist,
  * Also see the convenience function
  * gst_pb_utils_add_codec_description_to_tag_list().
  *
- * Returns: a newly-allocated description string, or NULL on error. Free
+ * Returns: (nullable): a newly-allocated description string, or NULL on error. Free
  *          string with g_free() when not needed any longer.
  */
 gchar *
@@ -1305,7 +1302,8 @@ pb_utils_get_file_extension_from_caps (const GstCaps * caps)
 gchar *
 gst_pb_utils_get_file_extension_from_caps (const GstCaps * caps)
 {
-  return g_strdup (pb_utils_get_file_extension_from_caps (caps));
+  const gchar *extension = pb_utils_get_file_extension_from_caps (caps);
+  return extension ? g_strdup (extension) : NULL;
 }
 
 /**
diff --git a/subprojects/gst-plugins-base/gst-libs/gst/pbutils/encoding-profile.c b/subprojects/gst-plugins-base/gst-libs/gst/pbutils/encoding-profile.c
index 66e404cfe0..11bbbf3f6d 100644
--- a/subprojects/gst-plugins-base/gst-libs/gst/pbutils/encoding-profile.c
+++ b/subprojects/gst-plugins-base/gst-libs/gst/pbutils/encoding-profile.c
@@ -515,7 +515,7 @@ gst_encoding_profile_class_init (GstEncodingProfileClass * klass)
  * gst_encoding_profile_get_name:
  * @profile: a #GstEncodingProfile
  *
- * Returns: the name of the profile, can be %NULL.
+ * Returns: (nullable): the name of the profile, can be %NULL.
  */
 const gchar *
 gst_encoding_profile_get_name (GstEncodingProfile * profile)
@@ -529,7 +529,7 @@ gst_encoding_profile_get_name (GstEncodingProfile * profile)
  * gst_encoding_profile_get_description:
  * @profile: a #GstEncodingProfile
  *
- * Returns: the description of the profile, can be %NULL.
+ * Returns: (nullable): the description of the profile, can be %NULL.
  */
 const gchar *
 gst_encoding_profile_get_description (GstEncodingProfile * profile)
@@ -543,7 +543,7 @@ gst_encoding_profile_get_description (GstEncodingProfile * profile)
  * gst_encoding_profile_get_format:
  * @profile: a #GstEncodingProfile
  *
- * Returns: (transfer full): the #GstCaps corresponding to the media format used
+ * Returns: (transfer full): (nullable): the #GstCaps corresponding to the media format used
  * in the profile. Unref after usage.
  */
 GstCaps *
@@ -558,7 +558,7 @@ gst_encoding_profile_get_format (GstEncodingProfile * profile)
  * gst_encoding_profile_get_preset:
  * @profile: a #GstEncodingProfile
  *
- * Returns: the name of the #GstPreset to be used in the profile.
+ * Returns: (nullable): the name of the #GstPreset to be used in the profile.
  * This is the name that has been set when saving the preset.
  */
 const gchar *
@@ -573,7 +573,7 @@ gst_encoding_profile_get_preset (GstEncodingProfile * profile)
  * gst_encoding_profile_get_preset_name:
  * @profile: a #GstEncodingProfile
  *
- * Returns: the name of the #GstPreset factory to be used in the profile.
+ * Returns: (nullable): the name of the #GstPreset factory to be used in the profile.
  */
 const gchar *
 gst_encoding_profile_get_preset_name (GstEncodingProfile * profile)
@@ -618,7 +618,7 @@ gst_encoding_profile_is_enabled (GstEncodingProfile * profile)
  * gst_encoding_profile_get_restriction:
  * @profile: a #GstEncodingProfile
  *
- * Returns: (transfer full): The restriction #GstCaps to apply before the encoder
+ * Returns: (transfer full) (nullable): The restriction #GstCaps to apply before the encoder
  * that will be used in the profile. The fields present in restriction caps are
  * properties of the raw stream (that is before encoding), such as height and
  * width for video and depth and sampling rate for audio. Does not apply to
@@ -637,7 +637,7 @@ gst_encoding_profile_get_restriction (GstEncodingProfile * profile)
 /**
  * gst_encoding_profile_set_name:
  * @profile: a #GstEncodingProfile
- * @name: (allow-none): the name to set on the profile
+ * @name: (nullable): the name to set on the profile
  *
  * Set @name as the given name for the @profile. A copy of @name will be made
  * internally.
@@ -654,7 +654,7 @@ gst_encoding_profile_set_name (GstEncodingProfile * profile, const gchar * name)
 /**
  * gst_encoding_profile_set_description:
  * @profile: a #GstEncodingProfile
- * @description: (allow-none): the description to set on the profile
+ * @description: (nullable): the description to set on the profile
  *
  * Set @description as the given description for the @profile. A copy of
  * @description will be made internally.
@@ -782,7 +782,7 @@ gst_encoding_profile_set_preset (GstEncodingProfile * profile,
 /**
  * gst_encoding_profile_set_preset_name:
  * @profile: a #GstEncodingProfile
- * @preset_name: (allow-none): The name of the preset to use in this @profile.
+ * @preset_name: (nullable): The name of the preset to use in this @profile.
  *
  * Sets the name of the #GstPreset's factory to be used in the profile.
  */
@@ -833,7 +833,7 @@ gst_encoding_profile_set_enabled (GstEncodingProfile * profile,
 /**
  * gst_encoding_profile_set_restriction:
  * @profile: a #GstEncodingProfile
- * @restriction: (allow-none) (transfer full): the restriction to apply
+ * @restriction: (nullable) (transfer full): the restriction to apply
  *
  * Set the restriction #GstCaps to apply before the encoder
  * that will be used in the profile. See gst_encoding_profile_get_restriction()
@@ -1279,11 +1279,11 @@ common_creation (GType objtype, GstCaps * format, const gchar * preset,
 
 /**
  * gst_encoding_container_profile_new:
- * @name: (allow-none): The name of the container profile, can be %NULL
- * @description: (allow-none): The description of the container profile,
+ * @name: (nullable): The name of the container profile, can be %NULL
+ * @description: (nullable): The description of the container profile,
  *     can be %NULL
  * @format: (transfer none): The format to use for this profile
- * @preset: (allow-none): The preset to use for this profile.
+ * @preset: (nullable): The preset to use for this profile.
  *
  * Creates a new #GstEncodingContainerProfile.
  *
@@ -1303,8 +1303,8 @@ gst_encoding_container_profile_new (const gchar * name,
 /**
  * gst_encoding_video_profile_new:
  * @format: (transfer none): the #GstCaps
- * @preset: (allow-none): the preset(s) to use on the encoder, can be %NULL
- * @restriction: (allow-none): the #GstCaps used to restrict the input to the encoder, can be
+ * @preset: (nullable): the preset(s) to use on the encoder, can be %NULL
+ * @restriction: (nullable): the #GstCaps used to restrict the input to the encoder, can be
  * NULL. See gst_encoding_profile_get_restriction() for more details.
  * @presence: the number of time this stream must be used. 0 means any number of
  *  times (including never)
@@ -1334,8 +1334,8 @@ gst_encoding_video_profile_new (GstCaps * format, const gchar * preset,
 /**
  * gst_encoding_audio_profile_new:
  * @format: (transfer none): the #GstCaps
- * @preset: (allow-none): the preset(s) to use on the encoder, can be %NULL
- * @restriction: (allow-none): the #GstCaps used to restrict the input to the encoder, can be
+ * @preset: (nullable): the preset(s) to use on the encoder, can be %NULL
+ * @restriction: (nullable): the #GstCaps used to restrict the input to the encoder, can be
  * NULL. See gst_encoding_profile_get_restriction() for more details.
  * @presence: the number of time this stream must be used. 0 means any number of
  *  times (including never)
@@ -1447,6 +1447,8 @@ gst_encoding_profile_get_type_nick (GstEncodingProfile * profile)
     return "video";
   if (GST_IS_ENCODING_AUDIO_PROFILE (profile))
     return "audio";
+
+  g_assert_not_reached ();
   return NULL;
 }
 
@@ -1492,7 +1494,7 @@ gst_encoding_container_profile_has_video (GstEncodingContainerProfile * profile)
  * gst_encoding_profile_get_file_extension:
  * @profile: a #GstEncodingProfile
  *
- * Returns: a suitable file extension for @profile, or NULL.
+ * Returns: (nullable): a suitable file extension for @profile, or NULL.
  */
 const gchar *
 gst_encoding_profile_get_file_extension (GstEncodingProfile * profile)
@@ -1596,13 +1598,13 @@ done:
 /**
  * gst_encoding_profile_find:
  * @targetname: (transfer none): The name of the target
- * @profilename: (transfer none) (allow-none): The name of the profile, if %NULL
+ * @profilename: (transfer none) (nullable): The name of the profile, if %NULL
  * provided, it will default to the encoding profile called `default`.
- * @category: (transfer none) (allow-none): The target category. Can be %NULL
+ * @category: (transfer none) (nullable): The target category. Can be %NULL
  *
  * Find the #GstEncodingProfile with the specified name and category.
  *
- * Returns: (transfer full): The matching #GstEncodingProfile or %NULL.
+ * Returns: (transfer full) (nullable): The matching #GstEncodingProfile or %NULL.
  */
 GstEncodingProfile *
 gst_encoding_profile_find (const gchar * targetname, const gchar * profilename,
@@ -2266,7 +2268,7 @@ create_stream_profile_recurse (GstEncodingProfile * toplevel,
  * #GstDiscovererInfo. Streams other than audio or video (eg,
  * subtitles), are currently ignored.
  *
- * Returns: (transfer full): The new #GstEncodingProfile or %NULL.
+ * Returns: (transfer full) (nullable): The new #GstEncodingProfile or %NULL.
  */
 GstEncodingProfile *
 gst_encoding_profile_from_discoverer (GstDiscovererInfo * info)
diff --git a/subprojects/gst-plugins-base/gst-libs/gst/pbutils/encoding-target.c b/subprojects/gst-plugins-base/gst-libs/gst/pbutils/encoding-target.c
index 5351c8a574..bd9dcdf302 100644
--- a/subprojects/gst-plugins-base/gst-libs/gst/pbutils/encoding-target.c
+++ b/subprojects/gst-plugins-base/gst-libs/gst/pbutils/encoding-target.c
@@ -198,7 +198,7 @@ gst_encoding_target_get_description (GstEncodingTarget * target)
  * gst_encoding_target_get_path:
  * @target: a #GstEncodingTarget
  *
- * Returns: (transfer none): The path to the @target file.
+ * Returns: (transfer none) (nullable) (type filename): The path to the @target file.
  *
  * Since: 1.18
  */
@@ -227,7 +227,7 @@ gst_encoding_target_get_profiles (GstEncodingTarget * target)
  * @target: a #GstEncodingTarget
  * @name: the name of the profile to retrieve
  *
- * Returns: (transfer full): The matching #GstEncodingProfile, or %NULL.
+ * Returns: (nullable) (transfer full): The matching #GstEncodingProfile, or %NULL.
  */
 GstEncodingProfile *
 gst_encoding_target_get_profile (GstEncodingTarget * target, const gchar * name)
@@ -309,7 +309,7 @@ validate_name (const gchar * name)
  * *can* be a application or user specific category if
  * needed.
  *
- * Returns: (transfer full): The newly created #GstEncodingTarget or %NULL if
+ * Returns: (transfer full) (nullable): The newly created #GstEncodingTarget or %NULL if
  * there was an error.
  */
 
@@ -898,7 +898,7 @@ gst_encoding_target_subload (gchar * path, const gchar * category,
  * @name: the name of the #GstEncodingTarget to load (automatically
  * converted to lower case internally as capital letters are not
  * valid for target names).
- * @category: (allow-none): the name of the target category, like
+ * @category: (nullable): the name of the target category, like
  * #GST_ENCODING_CATEGORY_DEVICE. Can be %NULL
  * @error: If an error occurred, this field will be filled in.
  *
@@ -1280,7 +1280,7 @@ merge_targets (GList * res, GList * extra)
 
 /**
  * gst_encoding_list_all_targets:
- * @categoryname: (allow-none): The category, for ex: #GST_ENCODING_CATEGORY_DEVICE.
+ * @categoryname: (nullable): The category, for ex: #GST_ENCODING_CATEGORY_DEVICE.
  * Can be %NULL.
  *
  * List all available #GstEncodingTarget for the specified category, or all categories
diff --git a/subprojects/gst-plugins-base/gst-libs/gst/pbutils/gstdiscoverer-types.c b/subprojects/gst-plugins-base/gst-libs/gst/pbutils/gstdiscoverer-types.c
index ac3e05bc50..f490ec20e0 100644
--- a/subprojects/gst-plugins-base/gst-libs/gst/pbutils/gstdiscoverer-types.c
+++ b/subprojects/gst-plugins-base/gst-libs/gst/pbutils/gstdiscoverer-types.c
@@ -609,7 +609,7 @@ gst_discoverer_stream_info_get_stream_type_nick (GstDiscovererStreamInfo * info)
  * gst_discoverer_stream_info_get_previous:
  * @info: a #GstDiscovererStreamInfo
  *
- * Returns: (transfer full): the previous #GstDiscovererStreamInfo in a chain.
+ * Returns: (transfer full) (nullable): the previous #GstDiscovererStreamInfo in a chain.
  * %NULL for starting points. Unref with #gst_discoverer_stream_info_unref
  * after usage.
  */
@@ -627,7 +627,7 @@ gst_discoverer_stream_info_get_previous (GstDiscovererStreamInfo * info)
  * gst_discoverer_stream_info_get_next:
  * @info: a #GstDiscovererStreamInfo
  *
- * Returns: (transfer full): the next #GstDiscovererStreamInfo in a chain. %NULL
+ * Returns: (transfer full) (nullable): the next #GstDiscovererStreamInfo in a chain. %NULL
  * for final streams.
  * Unref with #gst_discoverer_stream_info_unref after usage.
  */
@@ -646,7 +646,7 @@ gst_discoverer_stream_info_get_next (GstDiscovererStreamInfo * info)
  * gst_discoverer_stream_info_get_caps:
  * @info: a #GstDiscovererStreamInfo
  *
- * Returns: (transfer full): the #GstCaps of the stream. Unref with
+ * Returns: (transfer full) (nullable): the #GstCaps of the stream. Unref with
  * #gst_caps_unref after usage.
  */
 GstCaps *
@@ -663,7 +663,7 @@ gst_discoverer_stream_info_get_caps (GstDiscovererStreamInfo * info)
  * gst_discoverer_stream_info_get_tags:
  * @info: a #GstDiscovererStreamInfo
  *
- * Returns: (transfer none): the tags contained in this stream. If you wish to
+ * Returns: (transfer none) (nullable): the tags contained in this stream. If you wish to
  * use the tags after the life-time of @info you will need to copy them.
  */
 const GstTagList *
@@ -678,7 +678,7 @@ gst_discoverer_stream_info_get_tags (GstDiscovererStreamInfo * info)
  * gst_discoverer_stream_info_get_toc:
  * @info: a #GstDiscovererStreamInfo
  *
- * Returns: (transfer none): the TOC contained in this stream. If you wish to
+ * Returns: (transfer none) (nullable): the TOC contained in this stream. If you wish to
  * use the TOC after the life-time of @info you will need to copy it.
  */
 const GstToc *
@@ -712,7 +712,7 @@ gst_discoverer_stream_info_get_stream_id (GstDiscovererStreamInfo * info)
  * Deprecated: This functions is deprecated since version 1.4, use
  * #gst_discoverer_info_get_missing_elements_installer_details
  *
- * Returns: (transfer none): additional information regarding the stream (for
+ * Returns: (transfer none) (nullable): additional information regarding the stream (for
  * example codec version, profile, etc..). If you wish to use the #GstStructure
  * after the life-time of @info you will need to copy it.
  */
@@ -772,7 +772,7 @@ gst_discoverer_container_info_get_streams (GstDiscovererContainerInfo * info)
  * gst_discoverer_container_info_get_tags:
  * @info: a #GstDiscovererStreamInfo
  *
- * Returns: (transfer none): tags specific to the given container. If you wish to use
+ * Returns: (transfer none) (nullable): tags specific to the given container. If you wish to use
  * the tags after the life-time of @info, you will need to copy them.
  *
  * Since: 1.20
@@ -854,7 +854,7 @@ AUDIO_INFO_ACCESSOR_CODE (max_bitrate, guint, 0);
  * gst_discoverer_audio_info_get_language:
  * @info: a #GstDiscovererAudioInfo
  *
- * Returns: the language of the stream, or NULL if unknown.
+ * Returns: (nullable): the language of the stream, or NULL if unknown.
  */
 
 AUDIO_INFO_ACCESSOR_CODE (language, const gchar *, NULL);
@@ -987,7 +987,7 @@ gst_discoverer_video_info_is_image (const GstDiscovererVideoInfo * info)
  * gst_discoverer_subtitle_info_get_language:
  * @info: a #GstDiscovererSubtitleInfo
  *
- * Returns: the language of the stream, or NULL if unknown.
+ * Returns: (nullable): the language of the stream, or NULL if unknown.
  */
 
 SUBTITLE_INFO_ACCESSOR_CODE (language, const gchar *, NULL);
@@ -1022,7 +1022,7 @@ DISCOVERER_INFO_ACCESSOR_CODE (result, GstDiscovererResult, GST_DISCOVERER_OK);
  * gst_discoverer_info_get_stream_info:
  * @info: a #GstDiscovererInfo
  *
- * Returns: (transfer full): the structure (or topology) of the URI as a
+ * Returns: (transfer full) (nullable): the structure (or topology) of the URI as a
  * #GstDiscovererStreamInfo.
  * This structure can be traversed to see the original hierarchy. Unref with
  * gst_discoverer_stream_info_unref() after usage.
@@ -1098,7 +1098,7 @@ DISCOVERER_INFO_ACCESSOR_CODE (live, gboolean, FALSE);
  * Deprecated: This functions is deprecated since version 1.4, use
  * #gst_discoverer_info_get_missing_elements_installer_details
  *
- * Returns: (transfer none): Miscellaneous information stored as a #GstStructure
+ * Returns: (transfer none) (nullable): Miscellaneous information stored as a #GstStructure
  * (for example: information about missing plugins). If you wish to use the
  * #GstStructure after the life-time of @info, you will need to copy it.
  */
@@ -1109,7 +1109,7 @@ DISCOVERER_INFO_ACCESSOR_CODE (misc, const GstStructure *, NULL);
  * gst_discoverer_info_get_tags:
  * @info: a #GstDiscovererInfo
  *
- * Returns: (transfer none): all tags contained in the URI. If you wish to use
+ * Returns: (transfer none) (nullable): all tags contained in the URI. If you wish to use
  * the tags after the life-time of @info, you will need to copy them.
  *
  * Deprecated: 1.20: Use gst_discoverer_{container,stream}_info_get_tags() instead.
@@ -1122,7 +1122,7 @@ DISCOVERER_INFO_ACCESSOR_CODE (tags, const GstTagList *, NULL);
  * gst_discoverer_info_get_toc:
  * @info: a #GstDiscovererInfo
  *
- * Returns: (transfer none): TOC contained in the URI. If you wish to use
+ * Returns: (transfer none) (nullable): TOC contained in the URI. If you wish to use
  * the TOC after the life-time of @info, you will need to copy it.
  */
 
diff --git a/subprojects/gst-plugins-base/gst-libs/gst/pbutils/gstdiscoverer.c b/subprojects/gst-plugins-base/gst-libs/gst/pbutils/gstdiscoverer.c
index bd5feeef72..5ffc72d3c1 100644
--- a/subprojects/gst-plugins-base/gst-libs/gst/pbutils/gstdiscoverer.c
+++ b/subprojects/gst-plugins-base/gst-libs/gst/pbutils/gstdiscoverer.c
@@ -2545,7 +2545,7 @@ gst_discoverer_discover_uri_async (GstDiscoverer * discoverer,
  * gst_discoverer_discover_uri:
  * @discoverer: A #GstDiscoverer
  * @uri: The URI to run on.
- * @err: (out) (allow-none): If an error occurred, this field will be filled in.
+ * @err: (out) (optional): If an error occurred, this field will be filled in.
  *
  * Synchronously discovers the given @uri.
  *
@@ -2607,7 +2607,7 @@ gst_discoverer_discover_uri (GstDiscoverer * discoverer, const gchar * uri,
  * gst_discoverer_new:
  * @timeout: timeout per file, in nanoseconds. Allowed are values between
  *     one second (#GST_SECOND) and one hour (3600 * #GST_SECOND)
- * @err: a pointer to a #GError. can be %NULL
+ * @err: (optional): a pointer to a #GError. can be %NULL
  *
  * Creates a new #GstDiscoverer with the provided timeout.
  *
diff --git a/subprojects/gst-plugins-base/gst-libs/gst/pbutils/missing-plugins.c b/subprojects/gst-plugins-base/gst-libs/gst/pbutils/missing-plugins.c
index 52540bce49..69330b933b 100644
--- a/subprojects/gst-plugins-base/gst-libs/gst/pbutils/missing-plugins.c
+++ b/subprojects/gst-plugins-base/gst-libs/gst/pbutils/missing-plugins.c
@@ -205,7 +205,7 @@ copy_and_clean_caps (const GstCaps * caps)
  * that a source element for a particular URI protocol is missing. This
  * function is mainly for use in plugins.
  *
- * Returns: (transfer full): a new #GstMessage, or NULL on error
+ * Returns: (transfer full): a new #GstMessage
  */
 GstMessage *
 gst_missing_uri_source_message_new (GstElement * element,
@@ -238,7 +238,7 @@ gst_missing_uri_source_message_new (GstElement * element,
  * that a sink element for a particular URI protocol is missing. This
  * function is mainly for use in plugins.
  *
- * Returns: (transfer full): a new #GstMessage, or NULL on error
+ * Returns: (transfer full): a new #GstMessage
  */
 GstMessage *
 gst_missing_uri_sink_message_new (GstElement * element, const gchar * protocol)
@@ -270,7 +270,7 @@ gst_missing_uri_sink_message_new (GstElement * element, const gchar * protocol)
  * that a certain required element is missing. This function is mainly for
  * use in plugins.
  *
- * Returns: (transfer full): a new #GstMessage, or NULL on error
+ * Returns: (transfer full): a new #GstMessage
  */
 GstMessage *
 gst_missing_element_message_new (GstElement * element,
@@ -302,7 +302,7 @@ gst_missing_element_message_new (GstElement * element,
  * that a decoder element for a particular set of (fixed) caps is missing.
  * This function is mainly for use in plugins.
  *
- * Returns: (transfer full): a new #GstMessage, or NULL on error
+ * Returns: (transfer full): a new #GstMessage
  */
 GstMessage *
 gst_missing_decoder_message_new (GstElement * element,
@@ -342,7 +342,7 @@ gst_missing_decoder_message_new (GstElement * element,
  * that an encoder element for a particular set of (fixed) caps is missing.
  * This function is mainly for use in plugins.
  *
- * Returns: (transfer full): a new #GstMessage, or NULL on error
+ * Returns: (transfer full): a new #GstMessage
  */
 GstMessage *
 gst_missing_encoder_message_new (GstElement * element,
@@ -433,7 +433,7 @@ missing_structure_get_caps_detail (const GstStructure * s, GstCaps ** p_caps)
  * This function is mainly for applications that call external plugin
  * installation mechanisms using one of the two above-mentioned functions.
  *
- * Returns: a newly-allocated detail string, or NULL on error. Free string
+ * Returns: (nullable): a newly-allocated detail string, or NULL on error. Free string
  *          with g_free() when not needed any longer.
  */
 gchar *
@@ -530,7 +530,7 @@ error:
  * describing a missing plugin, given a previously collected missing-plugin
  * message
  *
- * Returns: a newly-allocated description string, or NULL on error. Free
+ * Returns: a newly-allocated description string. Free
  *          string with g_free() when not needed any longer.
  */
 gchar *
@@ -691,7 +691,7 @@ gst_installer_detail_new (gchar * description, const gchar * type,
  * the case where the application knows exactly what kind of plugin it is
  * missing.
  *
- * Returns: a newly-allocated detail string, or NULL on error. Free string
+ * Returns: (transfer full): a newly-allocated detail string. Free string
  *          with g_free() when not needed any longer.
  */
 gchar *
@@ -719,7 +719,7 @@ gst_missing_uri_source_installer_detail_new (const gchar * protocol)
  * the case where the application knows exactly what kind of plugin it is
  * missing.
  *
- * Returns: a newly-allocated detail string, or NULL on error. Free string
+ * Returns: (transfer full): a newly-allocated detail string. Free string
  *          with g_free() when not needed any longer.
  */
 gchar *
@@ -747,7 +747,7 @@ gst_missing_uri_sink_installer_detail_new (const gchar * protocol)
  * the case where the application knows exactly what kind of plugin it is
  * missing.
  *
- * Returns: a newly-allocated detail string, or NULL on error. Free string
+ * Returns: (transfer full): a newly-allocated detail string. Free string
  *          with g_free() when not needed any longer.
  */
 gchar *
@@ -774,7 +774,7 @@ gst_missing_element_installer_detail_new (const gchar * factory_name)
  * the case where the application knows exactly what kind of plugin it is
  * missing.
  *
- * Returns: a newly-allocated detail string, or NULL on error. Free string
+ * Returns: (transfer full): a newly-allocated detail string. Free string
  *          with g_free() when not needed any longer.
  */
 gchar *
@@ -812,7 +812,7 @@ gst_missing_decoder_installer_detail_new (const GstCaps * decode_caps)
  * the case where the application knows exactly what kind of plugin it is
  * missing.
  *
- * Returns: a newly-allocated detail string, or NULL on error. Free string
+ * Returns: (transfer full): a newly-allocated detail string. Free string
  *          with g_free() when not needed any longer.
  */
 gchar *