Apex domains cannot have CNAME records.
Question and response
Question
dig 0.193.160.210.in-addr.arpa. NS
Answer
0.193.160.210.in-addr.arpa. 4 NS ns6-tk01.ocn.ad.jp.
0.193.160.210.in-addr.arpa. 4 NS ns6-tk02.ocn.ad.jp.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0