Tools and Sessions

Odd one, that I'm hoping someone can clear up for me.

I created a tool, and added setters for:
public void setRequest(HttpServletRequest request)
and
public void setSession(HttpSession session)

The session is being passed in as "null", but request.getSession() is 
returning the session I'd expect to get.

Any idea why this would be?

(Running under Google App Engine, which doesn't seem to cause a problem 
otherwise.  But you never know...)

Andy