[Zope] session invalidate - using python

[p.t.]

Using this simple script, I expected that a None should be returned 
(according to 
http://www.zope.org/Documentation/Books/ZopeBook/2_6Edition/Sessions.stx):

data = context.session_data_manager.getSessionData( create=0 )
data.invalidate()
return str( data )    # str() just to see None

But a result like this is allways returned:

id: 10360120480192202025, token: 54001646A0iEAi2X9io, contents: []

with id change at each call (token is the same because the browser is the 
same).
A new session is ceated at each hit (refresh) in the same browser: and the 
counter in the transient object container is increased accordingly.
I did put Data object timeout value in minutes = 1, but after 30 minuts a 
new hit increase the counter.

Does create=0 work properly?
Does invalidate() work at all?
What am I doing wrong?

Thanks for help.
	p.t.