Possible bug in IS.pm

[Charley Tiggs <..hidden..>]

The recent fix for IS.pm seems to have tanked the ability to submit a 
credit against a previous invoice.

Let's say an invoice was submitted and paid for 2 months ago.  Customer 
returns the product and wants a credit.  When I go in and try to submit 
a new invoice reversing the specific line items on the invoice, I get an 
error:

Error: menu.pl:88: Error: menu.pl:88: Error: Too many reversed items on 
an invoice

I know for a fact that there are NOT too many reversed items on the invoice.

Am I missing something important?

Charley