wnck_screen_get_windows() returns empty window list

[Ben Liblit <liblit cs wisc edu>]

(I hereby join the growing list of people asking libwnck questions on 
the Metacity development list, since it's really not clear where libwnck 
questions belong.  Please accept my apologies if this is not the 
appropriate forum.)

I'm trying to get a list of windows on the default screen.  However, the 
obvious combination of wnck_screen_get_default() plus 
wnck_screen_get_windows() returns a NULL GList:

------------------------------------------------------------------------

    int main(int argc, char *argv[])
    {
      gtk_init(&argc, &argv);
      WnckScreen * const screen = wnck_screen_get_default();
      GList * const windows = wnck_screen_get_windows(screen);

      printf("window list: %p\n", windows);

      g_list_free(windows);
      return 0;
    }

==>

    window list: (nil)

------------------------------------------------------------------------

I've tried the Python analogue and it behaves analogously, yielding an 
empty list:

------------------------------------------------------------------------

    import wnck
    windows = wnck.screen_get_default().get_windows()
    print 'window list:', windows

==>

    window list: []

------------------------------------------------------------------------

Am I missing something *really* obvious here?