Apex domains cannot have CNAME records.
Question and response
Question
dig 128-255.158.137.209.in-addr.arpa. NS
Answer
128-255.158.137.209.in-addr.arpa. 5 NS ns-508.awsdns-63.com.
128-255.158.137.209.in-addr.arpa. 5 NS ns-557.awsdns-05.net.
128-255.158.137.209.in-addr.arpa. 5 NS ns-1216.awsdns-24.org.
128-255.158.137.209.in-addr.arpa. 5 NS ns-1726.awsdns-23.co.uk.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0