Missing Controller

Error: User.usernameController could not be found.

Error: Create the class User.usernameController below in file: app/controllers/user.username_controller.php

<?php
class User.usernameController extends AppController {

        var $name = 'User.username';
}
?>

Notice: If you want to customize this error message, create app/views/errors/missing_controller.ctp