Status von device abfragen [fehler]

[Robert Schulze <magot666 web de>]

GdkWindow       *gdwin;
GdkDevice       *device;
GdkModifierType state;

device = gdk_device_get_core_pointer();
gdk_device_get_state(device, gdwin, NULL, &state);

   === error-msg ===
 (gmouse:1239): Gdk-CRITICAL **: file gdkinput-none.c: line 47 (gdk_device_get_state): assertion 
`GDK_IS_WINDOW (window)' failed
  
  ==================
 
 What's my mistake?