diff --git a/gtk/gtkoverlay.c b/gtk/gtkoverlay.c
index c77b7cc3b3..49abeff466 100644
--- a/gtk/gtkoverlay.c
+++ b/gtk/gtkoverlay.c
@@ -203,13 +203,20 @@ gtk_overlay_get_main_widget_allocation (GtkOverlay *overlay,
       main_alloc.width = gtk_widget_get_allocated_width (grandchild);
       main_alloc.height = gtk_widget_get_allocated_height (grandchild);
     }
-  else
+  else if (GTK_IS_WIDGET (main_widget))
     {
       main_alloc.x = 0;
       main_alloc.y = 0;
       main_alloc.width = gtk_widget_get_allocated_width (main_widget);
       main_alloc.height = gtk_widget_get_allocated_height (main_widget);
     }
+  else
+    {
+      main_alloc.x = 0;
+      main_alloc.y = 0;
+      main_alloc.width = 1;
+      main_alloc.height = 1;
+    }
 
   if (main_alloc_out)
     *main_alloc_out = main_alloc;