diff --git a/gio/gtestdbus.c b/gio/gtestdbus.c
index 9cfaadbf0..d362f9478 100644
--- a/gio/gtestdbus.c
+++ b/gio/gtestdbus.c
@@ -251,6 +251,16 @@ watcher_init (void)
           g_assert_not_reached ();
         }
 
+      /* flush streams to avoid buffers being duplicated in the child and
+       * flushed by both the child and parent later
+       *
+       * FIXME: This is a workaround for the fact that watch_parent() uses
+       * non-async-signal-safe API. See
+       * https://gitlab.gnome.org/GNOME/glib/-/issues/2322#note_1034330
+       */
+      fflush (stdout);
+      fflush (stderr);
+
       switch (fork ())
         {
         case -1: