public function getLastModified(int $id): DateTimeImmutable
    {
        $headers = $this->parent->requestHeaders('head', '/series/'.$id);

        if (array_key_exists('Last-Modified', $headers) && array_key_exists(0, $headers['Last-Modified'])) {
            $lastModified = DateTimeImmutable::createFromFormat(
                static::LAST_MODIFIED_FORMAT,
                $headers['Last-Modified'][0]
            );

            if ($lastModified === false) {
                throw ParseException::lastModified($headers['Last-Modified'][0]);
            }

            return $lastModified;
        }

        throw ParseException::missingHeader('Last-Modified');
    }