[Zope] zope product problem

[Dominique Lederer]

hi!

i´m just diving into zope product development and have troubles  
implementing the example "poll" product from the Zope Developer's Guide:

my class looks like this:

class PollProduct(SimpleItem, PropertyManager):
   ...

i´am initializing my class attributes like this:

def __init__(self, id, question, responses):
   self.id=id
   self._question = question
   self._responses = responses


before that i define

_properties=(
       {'id':'question','type':'string','mode':'w'},
       {'id':'responses','type':'lines','mode':'w'},
     )

and

manage_options=(
       #{'label' : 'Test', 'action' : ''},
     ) + SimpleItem.manage_options + PropertyManager.manage_options

if i click on the "Properties" Tab in the ZMI i get this error
Error Type: AttributeError
Error Value: question

strange...i compared my product with some existing products and can´t find  
my mistake, maybe you see it with one look

tnx in advance!