NO!   That's the whole point of this class.  If you know how to serialize something on *ONE* side of the wire, you may not know on the *OTHER*, so this is ok to "fail".  It returns an UnserializedValue in that case, and you still have a toString of it.   

Conceptually, I like that model, although the details of how these defaultSerializers exhibit themselves needs some work here.   But the answer to your question is DEFINITELY NOT.