gtk_grab_add giving Gdk-CRITICAL

["Matthew Allen" <list sydneyband com au>]

Given:

        GtkWidget *Wnd = gtk_window_new(GTK_WINDOW_POPUP);
        gtk_window_set_decorated(GTK_WINDOW(Wnd), FALSE);
        gtk_window_set_type_hint(GTK_WINDOW(Wnd), GDK_WINDOW_TYPE_HINT_DROPDOWN_MENU);
        gtk_widget_add_events(Wnd, gtk_widget_get_events(Wnd) | GDK_BUTTON_PRESS_MASK);

        g_signal_connect (G_OBJECT(Wnd),
                                "button-press-event",
                                G_CALLBACK(PopupButtonEvent),
                                this);
        gtk_grab_add(Wnd);


The gtk_grab_add asserts with:

        Gdk-CRITICAL **: gdk_window_get_window_type: assertion `GDK_IS_WINDOW (window)' failed.

I passed a valid GtkWidget to the grab add, and there is nothing in the docs to infer that what I'm doing is 
bad... so what do I need to do to fix this code?

Regards
--
Matthew Allen