Error Message: "Can't find variable: XPathContext"

I'm using Selenium as part of Zelenium for Zope. I'm using it with Plone 2.1 
to test a pretty straightforward application.

On the development box, my test passes. On the staging box, my test fails with 
the error "Can't find variable: XPathContext".

A snippet of the test:

clickAndWait     //input[@value='Save']
assertText       //div[@id='approved-members']/ul/li  
      --> Can't find variable: XPathContext

This appears to be a message from Selenium's machinery, but don't understand 
the problem--why would the returned page (a pretty-straightforward Plone 
page) cause this problem?

Any advice would be great.

- j.