diff --git a/libavformat/movenc.c b/libavformat/movenc.c
index dd0a31f17d..16107b1596 100644
--- a/libavformat/movenc.c
+++ b/libavformat/movenc.c
@@ -3154,12 +3154,16 @@ static int mov_write_header(AVFormatContext *s)
     /* Set the FRAGMENT flag if any of the fragmentation methods are
      * enabled. */
     if (mov->max_fragment_duration || mov->max_fragment_size ||
-        mov->mode == MODE_ISM ||
         mov->flags & (FF_MOV_FLAG_EMPTY_MOOV |
                       FF_MOV_FLAG_FRAG_KEYFRAME |
                       FF_MOV_FLAG_FRAG_CUSTOM))
         mov->flags |= FF_MOV_FLAG_FRAGMENT;
 
+    /* Set other implicit flags immediately */
+    if (mov->mode == MODE_ISM)
+        mov->flags |= FF_MOV_FLAG_EMPTY_MOOV | FF_MOV_FLAG_SEPARATE_MOOF |
+                      FF_MOV_FLAG_FRAGMENT;
+
     /* faststart: moov at the beginning of the file, if supported */
     if (mov->flags & FF_MOV_FLAG_FASTSTART) {
         if ((mov->flags & FF_MOV_FLAG_FRAGMENT) ||
@@ -3297,7 +3301,6 @@ static int mov_write_header(AVFormatContext *s)
                             FF_MOV_FLAG_FRAG_CUSTOM)) &&
             !mov->max_fragment_duration && !mov->max_fragment_size)
             mov->max_fragment_duration = 5000000;
-        mov->flags |= FF_MOV_FLAG_EMPTY_MOOV | FF_MOV_FLAG_SEPARATE_MOOF;
     }
 
     if (!(mov->flags & FF_MOV_FLAG_FRAGMENT)) {