drush @drupal.d7 genu --roles=medewerker generated errors

$ drush @drupal.d7 genu --roles=medewerker 10
WD user: PDOException: SQLSTATE[HY000]: General error: 1366 Incorrect integer value: 'medewerker' for column 'rid' at row 1: INSERT INTO {users_roles} (uid, rid) VALUES (:db_insert_placeholder_0, :db_insert_placeholder_1); Array                                                                                                                          [error]
(
    [:db_insert_placeholder_0] => 4
    [:db_insert_placeholder_1] => medewerker
)
 in user_save() (line 603 of /Users/clemens/Sites/drupal/d7/www/modules/user/user.module).

We can validate these as invalid or check the string value against rol names.