[Zope] Error Value: 'Set' object has no attribute 'count'

I am getting an error message when Zope is trying to evaluate the following: elif string.count(product_info['product']['options'],'lookup') == 1: The error message is: Error Value: 'Set' object has no attribute 'count' I have two instances of Zope running. On Zope 2.8.2-final, python 2.3.5, linux2, this does not occur. On Zope 2.8.7-1.el4.rf, python 2.3.4, linux2, it does occur. Could anyone suggest a solution? Thanks and regards Cameron