Hi all. It looks like I've hit something which is already known (I've found this reference: http://mail.zope.org/pipermail/zope-collector-monitor/2004-October/004316.ht... ) I've a custom standard_error_html page, made with a Zope Page Template. I'm trying to provide a custom __str__ for a custom exception. This works, most of the time, but in some cases I encounter the following error: TypeError: unbound method __str__() must be called with InvalidFolderId instance as first argument (got nothing instead) InvalidFolderId is the custom exception, of course. This is the code of the exception: class InvalidFolderId(Exception): def __init__(self, folder_id): self.folder_id = folder_id def __str__(self): return "folder id %s could not be found in the database" % self.folder_id folder_id is a number. What could be done to avoid this problem? Thank you in advance for the help. Regards Marco