Cannot edit negative Contribution amount

Occasionally we create a negative $ amount (i.e. -$10) Contribution because of a refund, error, overpayment or credit. This is necessary and helpful use-case for our bookkeeping. CiviCRM allows the negative Contribution to be created, but once it is created, there is an error "Please enter a valid amount" if you try to edit the Contribution afterward. Please allow negative values to be edited.

Bug verifiable on Demo site.
http://drupal.demo.civicrm.org/civicrm/contact/view/contribution?reset=1&id=16&cid=103&action=view&context=contribution&selectedChild=contribute