diff --git a/libavformat/movenc.c b/libavformat/movenc.c
index c370922c7d..946e79c5ac 100644
--- a/libavformat/movenc.c
+++ b/libavformat/movenc.c
@@ -4858,8 +4858,8 @@ static int mov_write_trun_tag(AVIOContext *pb, MOVMuxContext *mov,
         if (i > first && get_sample_flags(track, &track->cluster[i]) != track->default_sample_flags)
             flags |= MOV_TRUN_SAMPLE_FLAGS;
     }
-    if (!(flags & MOV_TRUN_SAMPLE_FLAGS) && track->entry > 0 &&
-         get_sample_flags(track, &track->cluster[0]) != track->default_sample_flags)
+    if (!(flags & MOV_TRUN_SAMPLE_FLAGS) && track->entry > first &&
+         get_sample_flags(track, &track->cluster[first]) != track->default_sample_flags)
         flags |= MOV_TRUN_FIRST_SAMPLE_FLAGS;
     if (track->flags & MOV_TRACK_CTTS)
         flags |= MOV_TRUN_SAMPLE_CTS;