TRACE ("(%p, %p, %lx, %lx)\n", hWnd, pfnSubclass, uIDSubclass, dwRef);
+ if (!hWnd || !pfnSubclass)
+ return FALSE;
+
/* Since the window procedure that we set here has two additional arguments,
* we can't simply set it as the new window procedure of the window. So we
* set our own window procedure and then calculate the other two arguments