SQL Error on account creation from webserver_auth

I receive this error when creating an account on D6 using 'webserver_auth':

* warning: array_fill() [function.array-fill]: Number of elements must be positive in /var/www/drupal-6.14/includes/database.inc on line 253.
* warning: implode() [function.implode]: Bad arguments. in /var/www/drupal-6.14/includes/database.inc on line 253.
* warning: array_keys() [function.array-keys]: The first argument should be an array in /var/www/drupal-6.14/modules/user/user.module on line 502.
* user warning: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near ')' at line 1 query: SELECT p.perm FROM role r INNER JOIN permission p ON p.rid = r.rid WHERE r.rid IN () in /var/www/drupal-6.14/modules/user/user.module on line 502.

The error does not occur when I create the account as an admin using 'Add user'.