public function sendBroadcastMessageToGroup (TypeInterface $message, $groupID)
    {
        $json = (new Formatter())->formatMessageForBroadcast($message);
        $json['filter']['group_id'] = (int)$groupID;

        $json = json_decode(
            $this->client->send(
                new Request(
                    "POST",
                    "https://api.weixin.qq.com/cgi-bin/message/mass/sendall",
                    [],
                    json_encode($json)
                )
            )->getBody()
        );

        if (! isset($json->msg_id)) {
            throw new Exception("bad response: expected property `msg_id`");
        }

        return $json->msg_id;
    }