How to send and receive custom data type in XML-RPC [Java]

Question

I am trying to learn basics of XML-RPC protocol and implementing sample services using Apache xml-rpc library by following documentation at http://ws.apache.org/xmlrpc/types.html.

So far I have successfully set up a Calculator service which receives two integers and returns sum of integers in response. I am able to call this service using Dynamic Proxy client. Both service and client classes are in same eclipse project on my machine.

But issue is coming when I am trying to return my own custom data type. Here is how classes look like.

Main Calculator class

public class CalculatorImpl implements Calculator {
 @Override
 public int sum(int num1, int num2) {
    return num1 + num2;
 }
 @Override
 public CalculatorResponse getResponse(int num1, int num2) {
    return new CalculatorResponse(num1, num2);
 }
}

Service Class

public static void main(String[] args) throws Exception {
    XmlRpcServlet servlet = new XmlRpcServlet();

    ServletWebServer webServer = new ServletWebServer(servlet, port);
    XmlRpcServerConfigImpl serverConfig =
            (XmlRpcServerConfigImpl) webServer.getXmlRpcServer().getConfig();
    serverConfig.setEnabledForExtensions(true);
    serverConfig.setEnabledForExceptions(true);
    webServer.start();
}

Dynamic Proxy Client

public CalculatorServiceDynamicProxyClient() throws Exception {
    BasicConfigurator.configure();

    XmlRpcClientConfigImpl clientConfig = new XmlRpcClientConfigImpl();
    clientConfig.setServerURL(new URL(SERVICE_STRING_ENDPOINT));
    clientConfig.setEnabledForExtensions(true);
    clientConfig.setEnabledForExceptions(true);

    XmlRpcClient client = new XmlRpcClient();
    client.setConfig(clientConfig);
    client.setTransportFactory(new XmlRpcCommonsTransportFactory(client));

    clientFactory = new ClientFactory(client);
}

public int getSum(int num1, int num2) throws XmlRpcException {
    Calculator cal = (Calculator) clientFactory.newInstance(Calculator.class);
    return cal.sum(num1, num2);
}

public CalculatorResponse getResponse(int num1, int num2) {
    Calculator cal = (Calculator) clientFactory.newInstance(Calculator.class);
    return cal.getResponse(num1, num2);
}

public static void main(String[] args) throws Exception {
    CalculatorServiceDynamicProxyClient client = new CalculatorServiceDynamicProxyClient();
    System.out.println(client.getSum(4, 5)); //works
    System.out.println(client.getResponse(4, 5)); //throws exception
}

CalculatorResponse object just holds two integers and implements Serializable.

Error: org.apache.xmlrpc.client.XmlRpcClientException: to parse server's response: Premature end of file.

I am using 3.1.3 version of xmlrpc-server and xmlrpc-client jar.

Any suggestions would be helpful.

Thanks