RMI help!

I'm having trouble connecting clients to an RMI server.

The server starts up without any problems.

server GameServer has been created on localhost:7777
 and bound in the registry to the name GameServer

But I've got a Player object that I want to connect to GameServer. Here's the code:

        GameServer server = null;
        
        try {
            server = (GameServer)Naming.lookup("rmi://" + serverMachine +
                    "/" + serverName);
        } catch (Exception e) {
            System.err.println("Client exception " + e);
            System.exit(1);
        }

I can't figure out why an exception is being thrown.

Client exception java.lang.ClassCastException: GameServer_Stub

any help on what it means?