Apex domains cannot have CNAME records.
Question and response
Question
dig 128-143.218.36.138.in-addr.arpa. NS
Answer
128-143.218.36.138.in-addr.arpa. 5 NS ns-1651.awsdns-14.co.uk.
128-143.218.36.138.in-addr.arpa. 5 NS ns-1076.awsdns-06.org.
128-143.218.36.138.in-addr.arpa. 5 NS ns-35.awsdns-04.com.
128-143.218.36.138.in-addr.arpa. 5 NS ns-587.awsdns-09.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0