Cannot set Closed Status in a ticket with Update Ticket API

Hi,

We are using Update Ticket API (http://docs.whmcs.com/API:Update_Ticket) to change the Status of the tickets.
All status like Open, Hold On, Answered, In Progress are working fine. We can change these status with the API. But we cannot set status 'Closed' of a ticket with this API. When set $postfields["status"] = "Closed"; we did not get any result message (succes or error) from API. Please tell us how can we update the status to 'Closed'?

Best Regards,
Santanu