diff --git a/libavformat/utils.c b/libavformat/utils.c
index f1610d6798..ac68a132c5 100644
--- a/libavformat/utils.c
+++ b/libavformat/utils.c
@@ -399,6 +399,9 @@ int avformat_open_input(AVFormatContext **ps, const char *filename,
     if (options)
         av_dict_copy(&tmp, *options, 0);
 
+    if (s->pb) // must be before any goto fail
+        s->flags |= AVFMT_FLAG_CUSTOM_IO;
+
     if ((ret = av_opt_set_dict(s, &tmp)) < 0)
         goto fail;