How can I prevent a custom field formula error from displaying?

I want to display the quantity remaining (qty ordered less qty fulfilled) on the lines of a sales order. I'm using the suggestion here:

with a small tweak of the formula to exclude closed lines (case when {isclosed} <> 'T' then {quantity}-{quantityfulfilled} else 0 end).

The problem I'm having is that though it works perfectly for sales orders once they've been saved, when a user is creating a new sales order this field displays an error apparently because one of the fields referenced in my formula doesn't exist yet for that transaction. Usually it is "ERROR: Field 'quantityfulfilled' Not Found". I've tried adding some null-value handling to my formula, like adding