protected function setRevNum($revNum)
    {
        if (!is_numeric($revNum)) {
            throw new InvalidArgumentException(sprintf(
                'Revision must be an integer, received %s.',
                (is_object($revNum) ? get_class($revNum) : gettype($revNum))
            ));
        }

        $this->revNum = (int)$revNum;

        return $this;
    }