Empty string in username attribute causes DB integrity constraint violation

Here's what happens when the string is set, but it's the empty string:

PDOException: SQLSTATE[23000]: Integrity constraint violation: 1062 Duplicate entry '' for key 'name': UPDATE {users} SET name=:db_update_placeholder_0 WHERE (uid = :db_condition_placeholder_0) ; Array ( [:db_update_placeholder_0] => [:db_condition_placeholder_0] => 1599 ) in simplesamlphp_auth_user_insert() (line 378 of /path/to/drupal/sites/all/modules/simplesamlphp_auth/simplesamlphp_auth.module).