diff --git a/libavcodec/parser.c b/libavcodec/parser.c
index 30373c7622..670680ea7c 100644
--- a/libavcodec/parser.c
+++ b/libavcodec/parser.c
@@ -285,6 +285,8 @@ int ff_combine_frame(ParseContext *pc, int next,
         return -1;
     }
 
+    av_assert0(next >= 0 || pc->buffer);
+
     *buf_size          =
     pc->overread_index = pc->index + next;