[Zope] FSSession newbie problem

[Marcello Lupo]

Hello,
i'm trying to use the FSSession to have a little shopping cart for
e-commerce.

I can initialize the dictionary named "cart" and it is created in the
datafile.
When i try to update it with the data from my form i get the error:

Zope Error

             Zope has encountered an error while publishing this
resource. 

             Error Type: AttributeError
             Error Value: 'None' object has no attribute 'update'


For curiosity i tried the example bundled in README.txt of the FSSEssion
and macically i obtain the same error.
this is the code i used:
<dtml-call FSSession>
<dtml-unless "FSSession.has_key('cart')">
<dtml-call "FSSession.set('cart',{})">
</dtml-unless>
<dtml-comment>
#####
<dtml-var "REQUEST.form"><br>
<dtml-in "REQUEST.form.items()">
<dtml-var sequence-item>
</dtml-in>
This is only to check that the form passes the values correctly
########
</dtml-comment>
<dtml-call "FSSession['carrello'].update(REQUEST.form)">

Can anyone help me?
Thanks
ATIS