diff --git a/libavcodec/vc1dec.c b/libavcodec/vc1dec.c
index 73816018ff..421d8f77a6 100644
--- a/libavcodec/vc1dec.c
+++ b/libavcodec/vc1dec.c
@@ -3039,7 +3039,7 @@ static av_cold int vc1_decode_init(AVCodecContext *avctx)
         }
 
         buf2 = av_mallocz(avctx->extradata_size + FF_INPUT_BUFFER_PADDING_SIZE);
-        if(start[0]) start++; // in WVC1 extradata first byte is its size
+        start = find_next_marker(start, end); // in WVC1 extradata first byte is its size, but can be 0 in mkv
         next = start;
         for(; next < end; start = next){
             next = find_next_marker(start + 4, end);