Write to Invoice Line Item

Hi all,

I am attempting to write a value in an Invoice Line Item (where the Invoice has previously been created), however, each time I write to a line item where the Item is a Discount, my script falls over with the following message:

You have attempted an invalid sublist or line item operation. You are either trying to cannot access a field on a non-existent line or you are trying to add or remove lines from a static sublist.

Apart from the fact that the message is grammatically incorrect, I have no issue writing to Line Items containing Charge Items, so I'm thinking that my code is therefore 'mostly' correct.  Here's my code snippet: