[Zope] Problem with Int properties

Hi, although I'am working with plone I guess i'm having a Zope-problem here... I created a ZCLass inheriting from Base Document, added some properties and added that to the portal_types. There are several different properties, mainly booleans and some int-files (which drive me nuts at the moment). Everything works fine, you can add my doktype, edit it and things get changed except the ints. If I change the values thru ZMI, they will be shown in plone correctly (therefore there probably is no bis misspelling), changing and submitting will not do anything (neither change to selected value nor set to zero always) Is there anything special one has to do with fields of type "int" I might be missing??? thanks in advance Jochen