SQL error when viewing the gradebook

So far I've done nothing but import the tables, install and enable the module, and when I go to /gradebook, I get:

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 ') ORDER BY u.name ASC' at line 1 query: SELECT u.uid, u.name FROM users u INNER JOIN users_roles r ON u.uid = r.uid WHERE u.status != 0 AND r.rid IN () ORDER BY u.name ASC in /Applications/MAMP/htdocs/soc/includes/database.mysql.inc on line 120.