[Zope] Variable sort in dtml-in

[Aaron Williamson]

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