protected function checkCharsAllowed() {
        $c = strlen($this->text);

        // Special case, if we have 10 digits, the last one can be X
        if ($c === 10) {
            if (array_search($this->text[9], $this->keys) === false && $this->text[9] !== 'X') {
                throw new CINParseException('isbn', 'The character \'' . $this->text[9] . '\' is not allowed.');
            }

            // Drop the last char
            $this->text = substr($this->text, 0, 9);
        }

        return parent::checkCharsAllowed();
    }