Cannot post guestbook entries when running Drupal from a subdirectory

Running Guestbook 1.1.6.3 from a Drupal installation not running from root directory, the module does not respect the $base_url set in the site's settings.php file.

In my case, site runs from http://sitename.com/drupal

As a result, line 173 which contains:

       url("/guestbook/$uid") . "\">";

Causes a page not found when the form is submitted. (The form returns to http://sitename.com/guestbook/$uid which is nonexistant.

Fixed in my install by changing line to:

      url("guestbook/$uid") . "\">";

Is this the proper fix?