Return value must be of type ?Illuminate\Mail\SentMessage, int returned

Hello! I have a problem with your DKIM implementation. I get the following error when I send a mail:

php artisan Mail:sendShootingbookRequest

   TypeError 

  SimonSchaufi\LaravelDKIM\Mailer::send(): Return value must be of type ?Illuminate\Mail\SentMessage, int returned

  at vendor\simonschaufi\laravel-dkim\src\Mailer.php:25
     21▕      */
     22▕     public function send($view, array $data = [], $callback = null): ?SentMessage
     23▕     {
     24▕         if ($view instanceof MailableContract) {
  ➜  25▕             return $this->sendMailable($view);
     26▕         }
     27▕
     28▕         // First we need to parse the view, which could either be a string or an array
     29▕         // containing both an HTML and plain text versions of the view which should

  1   vendor\laravel\framework\src\Illuminate\Mail\PendingMail.php:124
      SimonSchaufi\LaravelDKIM\Mailer::send(Object(App\Mail\sendopenshootingbookrequestMail))

  2   app\Http\Controllers\api\v1\SendMailsController.php:116
      Illuminate\Mail\PendingMail::send(Object(App\Mail\sendopenshootingbookrequestMail))

What does the system want from me? Where should I look for the error

Thank you

simonschaufi / laravel-dkim

Return value must be of type ?Illuminate\Mail\SentMessage, int returned #26