Forwarding unknown (complex) type "as is"

Feel like I am definitely misunderstanding something here.

I have a fairly complex object that I want to send/receive. Not keen to write serialisation/deserialization for it. The documentation states:

The Photon Server is able to forward unknown custom types "as is". This is why you don't need to register your types in the Photon Cloud.

However if I try to send the object using Peer.OpCustom, i get a cannot serialize() Exception.

Can anyone please clarify what is meant by the above documentation snippet?

Thanks
Vinnie.