public function testBeforeSendPerformedWithInvalidMessage()
 {
     $this->event = $this->getMockBuilder(\Swift_Events_SendEvent::class)->disableOriginalConstructor()->getMock();
     $this->event->expects($this->any())->method('getMessage')->will($this->returnValue(new \stdClass()));
     $this->message->expects($this->never())->method('attachSigner');
     $this->plugin->beforeSendPerformed($this->event);
 }