Empty page upon accessing list page for already subscribed list

When a user tries to access the path emf/_listid_, they will get an empty page when they are already subscribed. Only when they visit another page, they will see the message "You are not allowed to view this subscription form or are already subscribed to this list." This is because the way this situation is handled; when the page callback encounters this situation, it will only call drupal_set_message and return nothing. For a page to be displayed, the menu callback either needs to return the page content, or return a status code. I think in this case, the callback should return MENU_ACCESS_DENIED.