public function setQuantityUnit($value)
    {
        if (!empty($value) && strlen($value) > 8) {
            throw new \Genesis\Exceptions\ErrorParameter(
                sprintf(
                    'Item parameter quantity_unit is set to %s, but expected to be string with max length 8 characters',
                    $value
                )
            );
        }

        $this->quantity_unit = $value;
        return $this;
    }