diff --git a/libavcodec/h264.c b/libavcodec/h264.c
index a9a10513e3..2bde0fec2a 100644
--- a/libavcodec/h264.c
+++ b/libavcodec/h264.c
@@ -3764,7 +3764,7 @@ static int decode_nal_units(H264Context *h, const uint8_t *buf, int buf_size){
         int err;
 
         if(buf_index >= next_avc) {
-            if(buf_index >= buf_size) break;
+            if (buf_index >= buf_size - h->nal_length_size) break;
             nalsize = 0;
             for(i = 0; i < h->nal_length_size; i++)
                 nalsize = (nalsize << 8) | buf[buf_index++];