diff --git a/gdk/x11/gdkmonitor-x11.c b/gdk/x11/gdkmonitor-x11.c
index 341917ad8c..85e1924053 100644
--- a/gdk/x11/gdkmonitor-x11.c
+++ b/gdk/x11/gdkmonitor-x11.c
@@ -44,6 +44,9 @@ gdk_monitor_has_fullscreen_window (GdkMonitor *monitor)
     {
       surface = l->data;
 
+      if (!GDK_IS_TOPLEVEL (surface))
+        continue;
+
       if ((gdk_toplevel_get_state (GDK_TOPLEVEL (surface)) & GDK_SURFACE_STATE_FULLSCREEN) == 0)
         continue;