Apex domains cannot have CNAME records.
Question and response
Question
dig 144-159.16.162.212.in-addr.arpa. NS
Answer
144-159.16.162.212.in-addr.arpa. 5 NS ns3.adacor.net.
144-159.16.162.212.in-addr.arpa. 5 NS ns4.adacor.net.
144-159.16.162.212.in-addr.arpa. 5 NS ns1.adacor.net.
144-159.16.162.212.in-addr.arpa. 5 NS ns2.adacor.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0