StateServer not allowing WCF returned type to be stored in session variable

Question

Hello,

I'm making a call to one of our internal WCF services from within a 1.1 web application and returning a type which I then store in session.

The returned type contains data for the page and when the page renders I get the following message:

"Unable to serialize the session state. Please note that non-seriWializable objects or MarshalByRef objects are not permitted when session state mode is "StateServer" or "SQLServer".

I don't understand why I would not be able to stored a type returned from a service into a session variable.

If I changed the config to "InProc" then it works.

Appreciate your feedback.