Apex domains cannot have CNAME records.
Question and response
Question
dig 100.69.216.in-addr.arpa. NS
Answer
100.69.216.in-addr.arpa. 4 NS ns-1459.awsdns-54.org.
100.69.216.in-addr.arpa. 4 NS ns-2023.awsdns-60.co.uk.
100.69.216.in-addr.arpa. 4 NS ns-228.awsdns-28.com.
100.69.216.in-addr.arpa. 4 NS ns-889.awsdns-47.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0