diff --git a/can-halal.c b/can-halal.c
index 8d72e06..a99303a 100644
--- a/can-halal.c
+++ b/can-halal.c
@@ -260,7 +260,7 @@ uint8_t *ftcan_marshal_unsigned(uint8_t *data, uint64_t val, size_t num_bytes) {
     num_bytes = 8;
   }
 
-  for (size_t i = num_bytes - 1; i >= 0; i++) {
+  for (int i = num_bytes - 1; i >= 0; i--) {
     data[i] = val & 0xFF;
     val >>= 8;
   }