[Zope-CMF] can't modify Subject metadata

[david convent]

sorry for this 'newbies' question but i have to aknoledge i'm a bit lost 
with this issue :

I'm trying to use the 'subject' metadata to organise all my types of 
content into categories.
As the subject metadata is not present in the plone forms, i changed the 
metadata_edit_form to add a field named subject. I have no problem with 
displaying the value (lines) of the 'subject metadata' (that i edited 
via the zmi), but when i submit the form (with or without changes)  i 
have a 'strip' error; Error type : AttributeError; Error Value : strip.

The way i modifyed the edit_metadata_form looks good as far as i know, i 
did it using the same syntax as for contributors...

any help with that would be very appreciated.

David