[Zope] error with tal:condition="python:x in s"

Greetings. I was wondering if this is a bug, or if I'm just doing something wrong. I'm trying the following page template code: <span tal:condition="python:'string' in s"> </span> s is a list of strings. I'm trying to condition on membership of 'string' in the list s, but I get the following error: exceptions.TypeError on 'in ' requires character as left operand in "", at line 56, column 4 any help would be appreciated. regards, Christian Gromoll