Rails - Aug 2006 - storing object with no "marshal_dump is defined" in session

One of my classes uses the class GSL::Poly.  It seems from this error 
message that rails won''t let me store it in the session.  ANy ideas
what
i can do?

Rendering 
/usr/local/lib/ruby/gems/1.8/gems/actionpack-1.12.1/lib/action_controller/templates/rescues/layout.rhtml
(500 Internal Error)
no marshal_dump is defined for class GSL::Poly
/usr/local/lib/ruby/1.8/pstore.rb:348:in `dump''

Thanks
Chris

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