private function _validateCustomerId($id = null)
    {
        if (empty($id) || trim($id) == "") {
            throw new InvalidArgumentException(
            'expected customer id to be set'
            );
        }
        if (!preg_match('/^[0-9A-Za-z_-]+$/', $id)) {
            throw new InvalidArgumentException(
            $id . ' is an invalid customer id.'
            );
        }

    }