protected function asCurrencyStringFallback($value, $currency = null)
    {
        if ($currency === null) {
            if ($this->currencyCode === null) {
                throw new InvalidConfigException('The default currency code for the formatter is not defined.');
            }
            $currency = $this->currencyCode;
        }

        return $currency . ' ' . $this->asDecimalStringFallback($value, 2);
    }