Can't Add a New Domain

[amcintosh]

I tried to add "profmacinstr.morris.mn.us" and received
Zone failed validation test. Invalid TLD . ( / profmacinstr.morris.mn.us).

I current have "vima.austin.tx.us" registered.
I also fail when I try to add "ipv6.vima.austin.tx.us"

Just to test, I found I can register "billybob.com"  
I cannot register billybob.austin.tx.us

I did register "profmacinstr.com" and populated it with a few records.

[astronyu]

The error says "Invalid TLD", Top Level Domain
mn.us is TLD
morris.mn.us is a subdomain.
profmacinstr.morris.mn.us is sub subdomain