WebService objects are null after Post via Postman

Question

I have multiple post methods given by the interface of a .wsdl (After adding service-reference) file. The objects HeartbeatRequest and ErrorRequest are also defined by the .wsdl

    .
    .
    .
    [WebMethod]
    public HeartbeatResponse Heartbeat(HeartbeatRequest request)
    {
       return request.Heartbeat.MessageHeader.BuildHeartbeatResponse();
    }

    [WebMethod]
    public HeartbeatResponse Error(ErrorRequest request)
    {
       throw new NotImplementedException();
    }
    .
    .
    .

When i try to call the services via Postman the objects (HearbeatRequest, ErrorRequest) are null.

Example: Post: via Postman

After posting the request object is just null.

Does this has to do something with the serialization? If yes, what do i have to change