Apex domains cannot have CNAME records.
Question and response
Question
dig 40.99.82.in-addr.arpa. NS
Answer
40.99.82.in-addr.arpa. 2 NS a.ns.ip-only.net.
40.99.82.in-addr.arpa. 2 NS b.ns.ip-only.net.
40.99.82.in-addr.arpa. 2 NS c.ns.ip-only.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0