Apex domains cannot have CNAME records.
Question and response
Question
dig 128.51.121.27.in-addr.arpa. NS
Answer
128.51.121.27.in-addr.arpa. 5 NS ns002-oakoark3ak56of45.o.d-53.jp.
128.51.121.27.in-addr.arpa. 5 NS ns002-oakoark3ak56of45.o.d-53.info.
128.51.121.27.in-addr.arpa. 5 NS ns002-oakoark3ak56of45.o.d-53.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0