Change Default Admin-Password-Required Username

I recently changed the name of my home folder using the instructions at support.apple.com, everything worked just fine. The only small problem is this: Whenever the "Administrator Password Required" box pops up, it now has the wrong username in the "Username:" box by default. It contains the full name of the user instead of the short name.

For example:
The correct username is: Foo
The default username is: Foo Bar

Normally removing the last name isn't too difficult, but it has become slightly annoying.

How can I change this?

Thank You.