diff --git a/libavformat/bit.c b/libavformat/bit.c
index 7b807b9bc1..5d05da0f81 100644
--- a/libavformat/bit.c
+++ b/libavformat/bit.c
@@ -133,6 +133,9 @@ static int write_packet(AVFormatContext *s, AVPacket *pkt)
     GetBitContext gb;
     int i;
 
+    if (pkt->size != 10)
+        return AVERROR(EINVAL);
+
     avio_wl16(pb, SYNC_WORD);
     avio_wl16(pb, 8 * 10);