[pygtk] GTK_IS_WIDGET on subclass of a widget fails

[Jeffrey Finkelstein]

I'm new to Gtk. I tried to create a subclass of an HBox and add it to a 
window, but I get a GtkWarning telling me that the assertion 
`GTK_IS_WIDGET()' fails on the instance of my subclass.

For example:

===code===

from gtk import Window, HBox, WINDOW_TOPLEVEL, main

class FooBox(HBox):
     def __init__(self): pass

window = Window(WINDOW_TOPLEVEL)
window.add(FooBox())
window.show_all()
main()

===/code===

yields a warning at "window.add()", namely "GtkWarning: 
gtk_container_add: assertion `GTK_IS_WIDGET (widget)' failed". This 
causes the widget to not be displayed in the window.

I don't really understand why this is; am I missing a method call?

Thanks.
Jeffrey