Apex domains cannot have CNAME records.
Question and response
Question
dig 112-127.218.36.138.in-addr.arpa. NS
Answer
112-127.218.36.138.in-addr.arpa. 5 NS ns-1728.awsdns-24.co.uk.
112-127.218.36.138.in-addr.arpa. 5 NS ns-1522.awsdns-62.org.
112-127.218.36.138.in-addr.arpa. 5 NS ns-424.awsdns-53.com.
112-127.218.36.138.in-addr.arpa. 5 NS ns-845.awsdns-41.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0