Apex domains cannot have CNAME records.
Question and response
Question
dig 32-47.218.36.138.in-addr.arpa. NS
Answer
32-47.218.36.138.in-addr.arpa. 5 NS ns-1942.awsdns-50.co.uk.
32-47.218.36.138.in-addr.arpa. 5 NS ns-286.awsdns-35.com.
32-47.218.36.138.in-addr.arpa. 5 NS ns-574.awsdns-07.net.
32-47.218.36.138.in-addr.arpa. 5 NS ns-1127.awsdns-12.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0