public function setRequestOrder($requestOrder)
    {
        if (!is_string($requestOrder)) {
            throw new InvalidArgumentException('Expected requestOrder to be a string');
        }

        if (empty($requestOrder)) {
            throw new InvalidArgumentException('An empty string is not a valid requestOrder value');
        }

        $this->requestOrder = array_unique(str_split(strtoupper($requestOrder), 1));
    }