diff --git a/gtk/gtktoggleaction.h b/gtk/gtktoggleaction.h
index df72534d72..5f72546326 100644
--- a/gtk/gtktoggleaction.h
+++ b/gtk/gtktoggleaction.h
@@ -56,7 +56,7 @@ struct _GtkToggleAction
 
   /*< private >*/
 
-  GtkToggleActionPrivate *private_data;
+  GtkToggleActionPrivate *GSEAL (private_data);
 };
 
 struct _GtkToggleActionClass