[Zope] Can't create ZClass constructors

Hi there. My ZOPE 2.6 installation has been working fine so far. However, since the last couple of days I can't create ZClasses constructor objects to my product. That is, I can create a ZClass, but only if I DO NOT check "Create constructor objects?" checkbox. If I DO check it and click on Add button, I get the following error: Error Type: KeyError Error Value: title This happens if I try to add ZClass in any product. Any clues how to fix this? Thanks, Daniel.