public function setNumPerPage($count)
    {
        if (!is_numeric($count)) {
            throw new InvalidArgumentException(
                'Number Per Page must be numeric.'
            );
        }

        $count = (int)$count;
        if ($count < 0) {
            throw new InvalidArgumentException(
                'Number Per Page must be greater than zero.'
            );
        }

        $this->numPerPage = $count;
        return $this;
    }