RPC Problem with HashMap containing ArrayList Value

I've an object which contains a HashMap<Long, Object> instance. I'm
finding that when I put a value of type ArrayList, RPC calls fail with
a SerializationException that starts with:

com.google.gwt.user.client.rpc.SerializationException
at
com.google.gwt.user.client.rpc.impl.SerializerBase.getTypeHandler(SerializerBase.java:
148)
...

Any ideas why and suggestions on what to do to get to my intended
process?

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