Apex domains cannot have CNAME records.
Question and response
Question
dig 12.in-addr.arpa. NS
Answer
12.in-addr.arpa. 5 NS dbru.br.ns.els-gms.att.net.
12.in-addr.arpa. 5 NS dmtu.mt.ns.els-gms.att.net.
12.in-addr.arpa. 5 NS cbru.br.ns.els-gms.att.net.
12.in-addr.arpa. 5 NS cmtu.mt.ns.els-gms.att.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0