Apex domains cannot have CNAME records.
Question and response
Question
dig 23.212.202.in-addr.arpa. NS
Answer
23.212.202.in-addr.arpa. 3 NS ns1.sphere.ad.jp.
23.212.202.in-addr.arpa. 3 NS ns2.sphere.ad.jp.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0