Apex domains cannot have CNAME records.
Question and response
Question
dig 224-27.17.154.195.in-addr.arpa. NS
Answer
224-27.17.154.195.in-addr.arpa. 5 NS ns-28.awsdns-03.com.
224-27.17.154.195.in-addr.arpa. 5 NS ns-1249.awsdns-28.org.
224-27.17.154.195.in-addr.arpa. 5 NS ns-851.awsdns-42.net.
224-27.17.154.195.in-addr.arpa. 5 NS ns-1573.awsdns-04.co.uk.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0