Can't create a MWI using primary extension for a user having an alternate extension in Unity Connection 10.5.1

[rchaddock]

In Unity Connection 10.5.1...

Why can't I create a MWI specifying UsePrimaryExtension (true) when the user as an Alternate Extension defined?

The CUPI interface responds with:
DATA_EXCEPTION

The CUALS interface responds with:
com.cisco.unity.cuals.CuDbException: A subquery has returned not exactly one row

BTW:  If I make the same request for the same user after deleting the Alternate Extension, the create MWI request works as expected.

Thanks,

--Rick