Help: argument type mismatch

I've been trying to get a callback to work, but have never done so before.  I'm 
currently hungup by the following error in the following line.  I've created 
simplified files to try and isolate the problem.  Anyone know what causes this 
exception?

>From Client.CallTheServer:
  l_nSuccess = m_ifaceServer.rmtRegisterClient(m_oCallback);
The error:
  trying to connect:TestServ
  Client.CallTheServer threw exception: java.lang.IllegalArgumentException: 
argument type mismatch
  argument type mismatch
  java.lang.IllegalArgumentException: argument type mismatch
          at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
          at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
          at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
          at java.lang.reflect.Method.invoke(Unknown Source)
          at sun.rmi.server.UnicastServerRef.dispatch(Unknown Source)
          at sun.rmi.transport.Transport$1.run(Unknown Source)
          at java.security.AccessController.doPrivileged(Native Method)
          at sun.rmi.transport.Transport.serviceCall(Unknown Source)
          at sun.rmi.transport.tcp.TCPTransport.handleMessages(Unknown Source)
          at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.run(Unknown 
Source)
          at java.lang.Thread.run(Unknown Source)
          at 
sun.rmi.transport.StreamRemoteCall.exceptionReceivedFromServer(Unknown Source)
          at sun.rmi.transport.StreamRemoteCall.executeCall(Unknown Source)
          at sun.rmi.server.UnicastRef.invoke(Unknown Source)
          at Server_Stub.rmtRegisterClient(Unknown Source)
          at Client.CallTheServer(Client.java:41)
          at Client.run(Client.java:21)
          at Client.main(Client.java:28)
  CallTheServer returned:0