Problem when user disables Guestbook

Here – http://drupal.org/node/214392 – you suggested a simple way to add the Guestbook page provided by Guestbook module in a pane of the user profile Panel.

But if an user disables his GuestBook, the "Page not found" page is rendered inside the pane, messing all the layout.

Is there an if statement that checks if user has guestbook enabled before trying to render the page?

I tried if (guestbook_page(arg(1))), but it did not solve the issue.

Thanks