diff --git a/gobject/gtype.c b/gobject/gtype.c
index a82144251..1fd31e87d 100644
--- a/gobject/gtype.c
+++ b/gobject/gtype.c
@@ -1006,7 +1006,10 @@ check_add_interface_L (GType instance_type,
     {
       g_warning ("attempting to add an interface (%s) to class (%s) after class_init",
                  NODE_NAME (iface), NODE_NAME (node));
-      return FALSE;
+
+      /* See https://bugzilla.gnome.org/show_bug.cgi?id=697229 */
+      if (!g_str_has_prefix (NODE_NAME (node), "gtkmm__CustomObject_"))
+        return FALSE;
     }
   tnode = lookup_type_node_I (NODE_PARENT_TYPE (iface));
   if (NODE_PARENT_TYPE (tnode) && !type_lookup_iface_entry_L (node, tnode))