GWT发送头到c#后端

我尝试像这样发送一个自定义头到后端:

RequestBuilder builder = new RequestBuilder(RequestBuilder.POST, url);
builder.setHeader("X-SessionID", "ABC123456");

并将通过:

public object AfterReceiveRequest(ref Message request, IClientChannel channel, InstanceContext instanceContext)
{
}

但是'Message'对象为空!我错在哪里?

谢谢!