Informix Database Problem

I am usin informix and i am unable to use case statement inside aggregate function. It always throws an error. following is the query I am using.

select nvl(count(case when (ccd.contacttype =1 and ccd.contactdisposition =2) then (ccd.sessionid) else 0 end),'ELSED (att)') as calls from contactcalldetail ccd inner join agentconnectiondetail acd on ccd.sessionid=acd.sessionid

Plz help me out.