diff --git a/libavformat/wtv.c b/libavformat/wtv.c
index 23b662d87161f2d81fa7521acd8aeea67149fdaf..8b927466cf83f3661742693927ed21b0473880be 100644
--- a/libavformat/wtv.c
+++ b/libavformat/wtv.c
@@ -472,6 +472,7 @@ static void get_attachment(AVFormatContext *s, AVIOContext *pb, int length)
     st->codec->codec_id   = AV_CODEC_ID_MJPEG;
     st->codec->codec_type = AVMEDIA_TYPE_ATTACHMENT;
     st->codec->extradata  = av_mallocz(filesize);
+    st->id = -1;
     if (!st->codec->extradata)
         goto done;
     st->codec->extradata_size = filesize;