[Zope] Stranger than Property

[Sven Hohage]

Hallo,
I've got a very strange error.
I'm working with external methods and folder proprties. The methods are
iterating over properties
and adding or deleting items.
Until friday everything was fine.Now I got this error when I try to view
the properties.
I'm not understanding why there is a mistake only by viewing the
properties.

->>>>
Error Type: TypeError
Error Value: len() of unsized object



Traceback (innermost last):
  File C:\Zope\lib\python\ZPublisher\Publish.py, line 222, in
publish_module
  File C:\Zope\lib\python\ZPublisher\Publish.py, line 187, in publish
  File C:\Zope\lib\python\Zope\__init__.py, line 221, in
zpublisher_exception_hook
    (Object: ElementWithAttributes)
  File C:\Zope\lib\python\ZPublisher\Publish.py, line 171, in publish
  File C:\Zope\lib\python\ZPublisher\mapply.py, line 160, in mapply
    (Object: manage_propertiesForm)
  File C:\Zope\lib\python\ZPublisher\Publish.py, line 112, in
call_object
    (Object: manage_propertiesForm)
  File C:\Zope\lib\python\App\special_dtml.py, line 121, in __call__
    (Object: manage_propertiesForm)
    (Info: C:\Zope\lib\python\OFS/properties.dtml)
  File C:\Zope\lib\python\DocumentTemplate\DT_String.py, line 502, in
__call__
    (Object: manage_propertiesForm)
  File C:\Zope\lib\python\DocumentTemplate\DT_In.py, line 691, in
renderwob
    (Object: propertyMap)
  File C:\Zope\lib\python\DocumentTemplate\DT_In.py, line 657, in
renderwob
    (Object: getProperty(id))
TypeError: (see above)