sort by function

[Gastone Penzo]

Hi,
i need to order by function like:

sort=sum(field1,field2,field3)+desc

but solr gives me this error:
Missing sort order.
why is this possible? i read that is possible to order by function, from 
version 1.3
(http://wiki.apache.org/solr/FunctionQuery#Sort_By_Function)

i use version 1.4

nobody has an idea?

thanx


Gastone