Apex domains cannot have CNAME records.
Question and response
Question
dig 168.209.62.in-addr.arpa. NS
Answer
168.209.62.in-addr.arpa. 3 NS d.ns.ip-only.net.
168.209.62.in-addr.arpa. 3 NS c.ns.ip-only.net.
168.209.62.in-addr.arpa. 3 NS a.ns.ip-only.net.
168.209.62.in-addr.arpa. 3 NS b.ns.ip-only.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0