GtkTextView getting coordinates (on Windows)

[martyn 2 russell bt com]

If I set the "motion_notify_event" for a GtkTextView (under windows), and
then move the mouse over that part of the screen, it does enter the callback
function I provide, but when I call the function
(gtk_text_view_window_to_buffer_coords):

  GtkTextWindowType text_window_type = 0;

  text_window_type = gtk_text_view_get_window_type(GTK_TEXT_VIEW(widget),
event->window);
  gtk_text_view_window_to_buffer_coords(GTK_TEXT_VIEW(widget),
text_window_type, pointer_x, pointer_y, &x, &y);

I get this error (repeated for each mouse motion over that window):

  11:37:46:Gtk: ** WARNING **  gtktextview.c:7315: can't get coords for
private windows

Any ideas why?

Regards,
Martyn