diff --git a/libavcodec/h263.c b/libavcodec/h263.c
index 2696102589..c60044fd37 100644
--- a/libavcodec/h263.c
+++ b/libavcodec/h263.c
@@ -2388,6 +2388,7 @@ void mpeg4_encode_picture_header(MpegEncContext * s, int picture_number)
     time_div= s->time/s->time_increment_resolution;
     time_mod= s->time%s->time_increment_resolution;
     time_incr= time_div - s->last_time_base;
+    assert(time_incr >= 0);
     while(time_incr--)
         put_bits(&s->pb, 1, 1);