Apex domains cannot have CNAME records.
Question and response
Question
dig 208.206.83.in-addr.arpa. NS
Answer
208.206.83.in-addr.arpa. 1 NS dns2.equant.net.
208.206.83.in-addr.arpa. 1 NS dns1.equant.net.
208.206.83.in-addr.arpa. 1 NS dns3.equant.net.
208.206.83.in-addr.arpa. 1 NS ns5.oleane.net.
208.206.83.in-addr.arpa. 1 NS ns4.oleane.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0