Missing GdkWindow from top level GtkWindow

[Gus Koppel <gtk spamkiller bytechase cx>]

Using GTK+ 2.2 on X I have a working top level GtkWindow and need to
refer to its GdkWindow. AFAIK gtk_widget_get_parent_window () is the
function to use. However, for my top level GtkWindow it always returns
NULL and emits "assertion 'widget->parent != NULL' failed". It's the
same in the initialization stage of my program as well as in all tested
event handlers (i.e. map, show, configure). How + where + when is the
GdkWindow of a top level GtkWindow supposed to be available?

I need the GdkWindow for gdk_window_set_icon () and associated pixmap
functions. gtk_window_set_icon () doesn't work either, maybe because it
misses its GdkWindow as well? Contents (widgets) of the window look and
function properly.