Field Total Amount Excl. VAT is readonly! Error during creation of SalesOrder Lines

Hi,
I am creating a sales order through a C# console application. When is try to create/update the salesorder lines of type "Item" then exception occurs {"Field Total Amount Excl. VAT is readonly!"} .

The code block is
salesOrder.SalesLines[0].Type = Type5.Item;
salesOrder.SalesLines[0].No = "70063";
salesOrder.SalesLines[0].Quantity = 1;

salesOrderService.Update(ref salesOrder); //Throws exception



Any type of help/suggestion would be really appreciative.