diff --git a/libavformat/smoothstreamingenc.c b/libavformat/smoothstreamingenc.c
index 66e6313934..3c050ca54e 100644
--- a/libavformat/smoothstreamingenc.c
+++ b/libavformat/smoothstreamingenc.c
@@ -340,7 +340,7 @@ static int ism_write_header(AVFormatContext *s)
         }
 
         av_dict_set_int(&opts, "ism_lookahead", c->lookahead_count, 0);
-        av_dict_set(&opts, "movflags", "frag_custom", 0);
+        av_dict_set(&opts, "movflags", "+frag_custom", 0);
         ret = avformat_write_header(ctx, &opts);
         av_dict_free(&opts);
         if (ret < 0) {