Error while updating handling cost after fullfillment

Hi All,

If I try to update handling cost with fulfillment this (hanlding cost) is getting updated(Fullfillment of salesorder ,handling cost are getting done).But once fulfillment is done , then if try to update handling cost it is giving error as
Error code : UNEXPECTED_ERROR
Error message : "An unexpected error occurred."

Can you please let me know what the problem is? Any help is highly appreciated.
I am using the following code to update the handling cost.

ItemFulfillment ifm = new ItemFulfillment (); //instantiate an ItemFulfillment
ifm.internalId = "118" //uniquely identify it by using the internal ID
ifm.handlingCost=2300.00;
ifm.handlingCostSpecified =true;