protected function assertLabel($label)
    {
        $utf8 = mb_convert_encoding($label, 'UTF-8', 'auto');

        if (!preg_match_all(static::STRICT_LABEL_PATTERN, $utf8, $matches)) {
            $message = sprintf('Could not serialize LTSV label = %s. It contains an illegal character.', $label);

            throw new \RuntimeException($message);
        }
    }