diff --git a/libavformat/movenc.c b/libavformat/movenc.c
index 68b3d74ef8..ba3c90453d 100644
--- a/libavformat/movenc.c
+++ b/libavformat/movenc.c
@@ -4357,10 +4357,9 @@ static int mov_write_header(AVFormatContext *s)
 
     /* faststart: moov at the beginning of the file, if supported */
     if (mov->flags & FF_MOV_FLAG_FASTSTART) {
-        if ((mov->flags & FF_MOV_FLAG_FRAGMENT) ||
-            (s->flags & AVFMT_FLAG_CUSTOM_IO)) {
+        if (mov->flags & FF_MOV_FLAG_FRAGMENT) {
             av_log(s, AV_LOG_WARNING, "The faststart flag is incompatible "
-                   "with fragmentation and custom IO, disabling faststart\n");
+                   "with fragmentation, disabling faststart\n");
             mov->flags &= ~FF_MOV_FLAG_FASTSTART;
         } else
             mov->reserved_moov_size = -1;