[Zope] commit_sub error in Product

[seb bacon]

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