protected function validateCombinationStockProduct($index, &$combination)
    {
        if (empty($combination['stock'])) {
            return null;
        }

        if (!is_numeric($combination['stock']) || strlen($combination['stock']) > 10) {
            $error = $this->translation->text('Only numeric values and no longer than @num characters', array('@num' => 10));
            $this->setError("combination.$index.stock", $error);
        }

        return !isset($error);
    }