Laravel方法Swift_Message :: __ toString（）不能抛出异常

Question

当我删除附件时我正在发送带有mailgun的电子邮件它工作得很好PHP版本是5.5.36而在另一个PHP版本为5.5.9的服务器上它可以正常工作。

$mail = Mail::send('emails.adviser', ['adviser' => $adviser , 'data' => $data , 'email_content' =>  $email_content ] , function ($message) use ($data, $adviser)   {
    $message->from($adviser->email, $adviser->first_name .' ' . $adviser->last_name);
    $message->to( $data['email'] )->subject('Re: Warburg Capital Group')->attach( asset('attachment/warburg_brochure.pdf'));
});