diff --git a/libavformat/jacosubenc.c b/libavformat/jacosubenc.c
index 324f2957fb..f0548bb282 100644
--- a/libavformat/jacosubenc.c
+++ b/libavformat/jacosubenc.c
@@ -24,7 +24,7 @@ static int jacosub_write_header(AVFormatContext *s)
     const AVCodecParameters *par = s->streams[0]->codecpar;
 
     if (par->extradata_size) {
-        avio_write(s->pb, par->extradata, par->extradata_size - 1);
+        avio_write(s->pb, par->extradata, par->extradata_size);
     }
     return 0;
 }