[Zope] Variable sort in dtml-in

In my search form, I have a selection box "in_sort_by" whose value can be "Category", "Field", or "Title". I am trying to make dtml-in sort by this value. I am using: <dtml-in SQL_search sort="_['in_sort_by']" size=list_size start=query_start> And I've tried other variations, but I always get an attribute error. The strange thing to me is that list_size is also a variable and it works fine, but I've tried just using in_sort_by and that doesn't work. What am I missing here? Many thanks, Aaron