[Zope] AttributeError: 'NoneType' object has no attribute 'lower'

when sorting using <...tal:repeat subsection python:sequence.sort (subsections,(('title','nocase','asc'),))"> I get the following error: Module DocumentTemplate.sequence.SortEx, line 134 in nocase AttributeError: 'NoneType' object has no attribute 'lower' When I switch the 'nocase' to 'cmp' the sort defaults to sorting by order of entry or something. Any help would be greatly appreciated. Thanks.