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

[kjcsb]

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