When an Administrator for a sub site removes a user from their sub site, I am trying to not only remove the user, but also completely delete them from the entire MU site. I have tried this:

add_action( 'remove_user_from_blog', 'custom_remove_user', 10,4 );
function custom_remove_user( $user_id ) {
     wpmu_delete_user( $user_id );
}

but i am not able to get it to work – it is not actually deleting the user and it fact when creating the user it is not being assigned to the proper sub site.

If I input an actual number of an existing user, the function works properly.

Any other suggestions for how to handle this?