Anonymous Commenting

I am unable to enable anonymous commenting on my Drupal site even though I have set the correct permissions. I am receiving an error on the comment settings page though, I'm not sure what is causing it though can anyone help? This is the error:

warning: array_keys() [function.array-keys]: The first argument should be an array in /var/www/modules/user/user.module on line 378.
warning: array_fill() [function.array-fill]: Number of elements must be positive in /var/www/modules/user/user.module on line 379.
warning: implode() [function.implode]: Bad arguments. in /var/www/modules/user/user.module on line 379.
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 DISTINCT(p.perm) FROM role r INNER JOIN permission p ON p.rid = r.rid WHERE r.rid IN () in /var/www/includes/database.mysql.inc on line 174.

Thanks