Thread:Project:Support desk/Changing rights for users with @ in their name

I am trying to assign rights to a user in my MediaWiki on the Special::UserRights page. The user names on this Wiki are of the form fred@xx.yy.com. If I have

$wgGroupPermissions['bureaucrat']['userrights-interwiki'] = true;

in my LocalSettings.php file, then I get

Enter a username:  fred@xx.yy.com Database uk.ibm.com' does not exist or is not local.

If I have

$wgGroupPermissions['bureaucrat']['userrights-interwiki'] = false;

I get:

You do not have permission to edit user rights on other wikis.

Versions:

Product 	Version MediaWiki 	1.16.1 PHP 	5.3.3 (apache2handler) MySQL 	5.0.77

Clearly the @ in my usernames is the problem - how can I work around this? Many thanks...