diff --git a/gtk/gtkcolorsel.h b/gtk/gtkcolorsel.h
index 4d2fa855ca..fedb029588 100644
--- a/gtk/gtkcolorsel.h
+++ b/gtk/gtkcolorsel.h
@@ -60,7 +60,7 @@ struct _GtkColorSelection
   GtkVBox parent_instance;
 
   /* < private_data > */
-  gpointer private_data;
+  gpointer GSEAL (private_data);
 };
 
 struct _GtkColorSelectionClass