[Zope] commit_sub error in Product

I'm writing a minimal product for which I'm getting a Error Type: AttributeError Error Value: commit_sub from the transaction machinery. The traceback's no help. The offending code, however is: class Foo(SimpleItem) def __init__(self, id, title, file): self.file = Image('file',title,'') self.file.manage_upload(file) Can anyone tell me what's wrong here? Cheers, seb