Custom Transaction Line Field and related issues

Hi there,

I want to have a transaction line field that is filled with a value depending on the itemid.

My journey so far:

I have made a custom transaction line field before that was picked up straight from an Item record field. So I decided to create a custom Item field first. I went to Validation & Defaulting, checked the 'formula' and put in:

CASE WHEN {itemid}="(item name)" THEN {description} ELSE {itemid} END

So this will then fill that field with whatever is in Description field if the item id is a certain phrase, otherwise it fills it with itemid. Well.. that didn't work. "invalid expression" for one item (didn't see a description field there, so may be empty?) and for another item it was simply empty (even though there was an itemid and a description.. so it should've used itemid for it).