protected function validateQuantityCart()
    {
        $field = 'quantity';

        if ($this->isExcluded($field)) {
            return null;
        }

        $value = $this->getSubmitted($field);

        if (!isset($value)) {

            if ($this->isUpdating()) {
                return null;
            }

            $value = 1;
        }

        $label = $this->translation->text('Quantity');

        if (!is_numeric($value)) {
            $this->setErrorNumeric($field, $label);
            return false;
        }

        if (strlen($value) > 2) {
            $this->setErrorLengthRange($field, $label, 1, 2);
            return false;
        }

        return true;
    }