Apex domains cannot have CNAME records.
Question and response
Question
dig 207.231.202.in-addr.arpa. NS
Answer
207.231.202.in-addr.arpa. 5 NS dns02.bekknet.ad.jp.
207.231.202.in-addr.arpa. 5 NS dns01.bekknet.ad.jp.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0