Apex domains cannot have CNAME records.
Question and response
Question
dig 96-27.240.172.163.in-addr.arpa. NS
Answer
96-27.240.172.163.in-addr.arpa. 4 NS ns-7.awsdns-00.com.
96-27.240.172.163.in-addr.arpa. 4 NS ns-1573.awsdns-04.co.uk.
96-27.240.172.163.in-addr.arpa. 4 NS ns-1292.awsdns-33.org.
96-27.240.172.163.in-addr.arpa. 4 NS ns-829.awsdns-39.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0