Item fulfillment using .NET code sample

I have downloaded sample CRM application from developer resources section and tried to implement partial item fulfillment. Here is the code:

private void SalesOrderFulfilling(int SoInternalId)          {              this.login(true);              InitializeRef SOref = new InitializeRef();              SOref.type = InitializeRefType.salesOrder;              SOref.internalId = SoInternalId.ToString();              SOref.typeSpecified = true;              InitializeRecord fulfillmentRecord = new InitializeRecord();              fulfillmentRecord.type = InitializeType.itemFulfillment;              fulfillmentRecord.reference = SOref;              ReadResponse IFinitResponse = _service.initialize(fulfillmentRecord);              ((ItemFulfillment)IFinitResponse.record).itemList.item[0].itemIsFulfilled = true;              ((ItemFulfillment)IFinitResponse.record).itemList.item[1].itemIsFulfilled = true;              WriteResponse fulfillmentUpdateResponse = _service.update(IFinitResponse.record);   // INVALID_KEY_OR_REF in response          }

Right now we can initialize Item Fulfillment, but cannot do the fulfilling operation. We receive the INVALID_KEY_OR_REF code in response.