diff --git a/libavcodec/mpeg4videoenc.c b/libavcodec/mpeg4videoenc.c
index 64cbecba9a..36dd3c90a2 100644
--- a/libavcodec/mpeg4videoenc.c
+++ b/libavcodec/mpeg4videoenc.c
@@ -242,6 +242,7 @@ void ff_clean_mpeg4_qscales(MpegEncContext *s){
 static inline void mpeg4_encode_dc(PutBitContext * s, int level, int n)
 {
 #if 1
+    /* DC will overflow if level is outside the [-255,255] range. */
     level+=256;
     if (n < 4) {
         /* luminance */