Colormap and drawing_area widget

[Luc Maurette sanofi-synthelabo com]

Hi everybody,
I have a little pb with gtk_drawing_area. I read tutorial and API
documentation but i don't find the answer : why i obtain this error message
when using the code below ?

*************Code*********************
GtkWidget *ZoneDessin;
GdkColormap *colormap;

ZoneDessin = gtk_drawing_area_new();
colormap = gdk_window_get_colormap(ZoneDessin->window);

*************Error***********************
Gdk-CRITICAL **: file gdkdraw.c: line 244 (gdk_drawable_get_colormap):
assertion `GDK_IS_COLORMAP (colormap)' failed

Thanks, Luc.