diff --git a/gtk/gtkaboutdialog.h b/gtk/gtkaboutdialog.h
index bf471044c3..02f2fbec03 100644
--- a/gtk/gtkaboutdialog.h
+++ b/gtk/gtkaboutdialog.h
@@ -48,7 +48,7 @@ struct _GtkAboutDialog
   GtkDialog parent_instance;
 
   /*< private >*/
-  gpointer private_data;
+  gpointer GSEAL (private_data);
 };
 
 struct _GtkAboutDialogClass