Apex domains cannot have CNAME records.
Question and response
Question
dig 91.7.211.in-addr.arpa. NS
Answer
91.7.211.in-addr.arpa. 1 NS ns010-2g9qdhup8irhm9sa.2.d-53.jp.
91.7.211.in-addr.arpa. 1 NS ns010-2g9qdhup8irhm9sa.2.d-53.net.
91.7.211.in-addr.arpa. 1 NS ns010-2g9qdhup8irhm9sa.2.d-53.info.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0