diff --git a/gdk/gdksurface.c b/gdk/gdksurface.c
index 4b485a012d..c32c17262b 100644
--- a/gdk/gdksurface.c
+++ b/gdk/gdksurface.c
@@ -2051,6 +2051,9 @@ update_cursor (GdkDisplay *display,
   GdkDeviceGrabInfo *grab;
   GdkCursor *cursor;
 
+  g_assert (display);
+  g_assert (device);
+
   pointer_info = _gdk_display_get_pointer_info (display, device);
   pointer_surface = pointer_info->surface_under_pointer;