我该如何解决此错误？为什么会给出错误？对于关系实例

Question

// ClientUser模型

public function devices()
{
    return $this->hasMany(UserDevice::class, 'as_user_id');
}

public function last_device(){
    $device = UserDevice::where('as_user_id',$this->id)->orderBy('id','desc')->first();
    return $device?$device:null ;
}

//控制器：

$user = ClientUser::where("id",$request->auth_user_id)->first();
$last_device = $user? $user->last_device : null;
dd(optional($last_device)->id);

//错误消息：

App \ Models \ Frontend \ ClientUser :: last_device必须返回一个关系实例