Adding roles by product purchase causes two watchdog entries

When I upgraded a buyer to add a new role on purchase i get two entries in the watchdog log.

Adding bill to role .
Adding bill to role subscriber.

The role being added is "subscriber", but it appears a blank watchdog entry gets added, cluttering up the log. I don't think there are any critical side effects from this.

Offending code starts around line 922:

   // If PAID - add user to role, else drop 'em
    if ($add_to_roles) {
      foreach($on_payment_roles as $rid => $rname) {
        if (!isset($u->roles[$rid])) {
          watchdog('ecommerce', t('Adding %name to role %role.', array('%name' => theme('placeholder', $u->name), '%role' => theme('placeholder', $roles[$rid]))));
          $u->roles[$rid] = '';
        }
      }
      user_save($u, array('roles' => $u->roles));
    }