diff --git a/libavformat/movenc.c b/libavformat/movenc.c
index 572e781fdc1049853fd7aae3791a6a10c08adeec..3ee54bf3f38cce257f89b2f76b0e8ac62817509d 100644
--- a/libavformat/movenc.c
+++ b/libavformat/movenc.c
@@ -4048,6 +4048,7 @@ static int mov_write_header(AVFormatContext *s)
     if (mov->flags & FF_MOV_FLAG_EMPTY_MOOV &&
         !(mov->flags & FF_MOV_FLAG_DELAY_MOOV)) {
         mov_write_moov_tag(pb, mov, s);
+        avio_flush(pb);
         mov->moov_written = 1;
         if (mov->flags & FF_MOV_FLAG_GLOBAL_SIDX)
             mov->reserved_header_pos = avio_tell(pb);