diff --git a/gtk/gtkdrawingarea.h b/gtk/gtkdrawingarea.h
index 553b9be97d..c40bc934a1 100644
--- a/gtk/gtkdrawingarea.h
+++ b/gtk/gtkdrawingarea.h
@@ -51,6 +51,9 @@ typedef struct _GtkDrawingAreaClass  GtkDrawingAreaClass;
 struct _GtkDrawingArea
 {
   GtkWidget widget;
+
+  /*< private >*/
+  gpointer dummy;
 };
 
 struct _GtkDrawingAreaClass