"Unexpected end of file from server" when adding serializable object to RemoteInvocat

[p.g.taboada]

I am adding metadata to a RemoteInvocation (using HttpInvoker).

As soon as I add an instance of

public class SessionInformation implements Serializable {
...
}

I get an "Unexpected end of file from server".

The class has 3 properties backed by 3 attributes (String, String and int) and a default constructor.
Really no magic at all.

Am I missing someting?