[Zope] External Methods triggers Zope restart

I am having a frustrating problem with External Methods. I have regressed to Zope 2.4.0 to try to resolve this. Basically, the following external method works: def helloWorld(): """A simple external method.""" return 'Hello World!' But the following method causes Zope to restart with no traceback: def helloWorld(): """A simple external method.""" a = 'Hello World!' return a If I use self.a instead of just a, the method works. I used th epython debugger, and the correct HTML is displayed in both cases. I don't find anything usefull in the -M log either. Has anyone seen anything like this, or have any ideas where I should look next. Thanks, Paul OS: HP-UX 10.20 Python 2.1.1 Zope 2.4.0