NullPointerException with CompositeService when calling method inherited from super-interface

[federicoon]

Following the example in the documentation, a composite server is created as:

    UserverService userService = ...;
    ContentService contentService = ...;
    BlackJackService blackJackService = ...;

    Object compositeService = ProxyUtil.createCompositeServiceProxy(
        this.getClass().getClassLoader(),
        new Object[] { userService, contentService, blackJackService},
        new Class<?>[] { UserService.class, ContentService.class, BlackJackService.class},
        true);

And then the corresponding client-side code is:

    UserverService userService = ProxyUtil.createClientProxy(
                UserverService.class.getClassLoader(), UserverService.class,
                jsonRpcClient, socket);
    ContentService contentService = ...;
    BlackJackService blackJackService = ...;

    Object compositeService = ProxyUtil.createCompositeServiceProxy(
        this.getClass().getClassLoader(),
        new Object[] { userService, contentService, blackJackService},
        new Class<?>[] { UserService.class, ContentService.class, BlackJackService.class},
        true);

    // now compositeService can be used as any of the above service, ie:
    User user = ((UserverService)compositService).createUser(...);
    Content content =  ((ContentService)compositService).getContent(...);
    Hand hand = ((BlackJackService)compositService).dealHand(...);  

Now, suppose both UserverService and ContentService extend a super-interface OperationService, which declares a method getOperation();
Then both calls:

    int id = ((UserverService)compositService).getOperation();
    int id = ((ContentService)compositService).getOperation();

will throw the following exception:

java.lang.NullPointerException
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:606)
at com.googlecode.jsonrpc4j.ProxyUtil$1.invoke(ProxyUtil.java:103)
at com.sun.proxy.$Proxy6.getOperation(Unknown Source)