Error when trying to reverse distribution lines (that are matched to purchase orders)

We are getting ready to go live with R12 and I came accross a problem.  My SR hasn't been helpful.  When trying to reverse or discard a line that is matched to a purchase order I get an error.  The system allowed me to discard a line (matched to a PO) on an invoice that had not been validated.   The invoices that have been validated all give the Error.  The Error is:  Cannot discard the invoice liine because a required call to tax calculation failed due to: &REASON. 

Has anyone came across this?  Is this intended functionality? We read somewhere this is standard functionality.  We are able to reverse lines currently in 11i that are validated and matched to PO's.