How do I Unreseve a PO that has an outstanding credit memo?

Hello,

I have a PO that had both a standard invoice and a credit invoice matched to it.  We want to make a change to the PO, so we reversed (unmatched) the two invoices and attempted to Unreserve the PO.  But, unresverving the PO fails with the following message, despite the fact that the PO invoices form shows both unmtached.

'Error:You cannot unreserve this document because there are outstanding credit memos'

Both the invoice and credit have checks associated with them.  Could this be the reason the system still considers the Credit as outstanding against the PO, and is not fully reversing it?