protected function validate(array $values, $operator)
    {
        if (!in_array($operator, array('=', '!='))) {
            return $this->translation->text('Unsupported operator');
        }

        if (count($values) != 1) {
            return $this->translation->text('@field has invalid value', array(
                '@field' => $this->translation->text('Condition')));
        }

        $scope = filter_var(reset($values), FILTER_VALIDATE_BOOLEAN, FILTER_NULL_ON_FAILURE);

        if (!isset($scope)) {
            return $this->translation->text('@field has invalid value', array(
                '@field' => $this->translation->text('Condition')));
        }

        return true;
    }