How do you apply a credit memo to a debit memo?

We had a user create a debit memo in error a couple of months ago.   She wants to apply a credit memo to reverse it.  So, she is trying it first in a test instance and she created a debit memo.   Then to apply the credit she did the following:

Brought up the debit memo in the Transactions form.

Went to  Actions=>Credit

Clicked Credit Balance

She got a pop-up form stating "The balance of the transaction has been credited. Please save your changes and complete the credit memo."

She clicked OK on the pop-up and in the Type field chose Credit Memo and clicked ok.