diff --git a/libavcodec/dvdsub_parser.c b/libavcodec/dvdsub_parser.c
index 07ed4f72fc..32a945ed65 100644
--- a/libavcodec/dvdsub_parser.c
+++ b/libavcodec/dvdsub_parser.c
@@ -46,6 +46,8 @@ static int dvdsub_parse(AVCodecParserContext *s,
 
     if (pc->packet_index == 0) {
         if (buf_size < 2 || AV_RB16(buf) && buf_size < 6) {
+            if (buf_size)
+                av_log(avctx, AV_LOG_DEBUG, "Parser input %d too small\n", buf_size);
             return buf_size;
         }
         pc->packet_len = AV_RB16(buf);