Combine Client; Merging an External Id caused merge to fail

Combine Client; Merging a Client with an External Id into a Client with no External Id caused the merge to fail with the following error message:

ERROR MESSAGE
=============
Failed to combine duplicated Client 'Lucas' with Client 'C100138'.

Failed to save Client.

Client 'C100138' External Id 'ex id for C100200' is invalid. The External Id already exists against another Client.


The External Id must be unique per Client. This error occurred as the External Id was being saved into the Client record before the redundant Client record was deleted.