Apex domains cannot have CNAME records.
Question and response
Question
dig 53.121.27.in-addr.arpa. NS
Answer
53.121.27.in-addr.arpa. 4 NS ns010-0cdmc93mlje5vnf8.0.d-53.jp.
53.121.27.in-addr.arpa. 4 NS ns010-0cdmc93mlje5vnf8.0.d-53.net.
53.121.27.in-addr.arpa. 4 NS ns010-0cdmc93mlje5vnf8.0.d-53.info.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0