[Zope] Can't override standard_error_message on NotFound?

I'm having some problems with standard_error_message on 2.2.0b2. According to the mailing list archives I should be able to override NotFound errors (I even cut-n-pasted http://www.zope.org/standard_error_message/view_source into my own standard_error_message in case I was missing something) but requesting a nonexistent object still gets me what appears to be a built-in page. Is there something else to do?