public function send()
    {
        if (empty($this->message)) {
            throw new RuntimeException('No message to send.');
        }

        $transformer = new Transformer();

        if ($this->message instanceof RawMessage) {
            $message = $this->message->get('content');
        } else {
            $content = $transformer->transform($this->message);
            $message = [
                'touser' => $this->to,
            ];

            if ($this->account) {
                $message['customservice'] = ['kf_account' => $this->account];
            }

            $message = array_merge($message, $content);
        }

        return $this->staff->send($message);
    }