Apex domains cannot have CNAME records.
Question and response
Question
dig 192-255.135.137.209.in-addr.arpa. NS
Answer
192-255.135.137.209.in-addr.arpa. 5 NS ns-1278.awsdns-31.org.
192-255.135.137.209.in-addr.arpa. 5 NS ns-248.awsdns-31.com.
192-255.135.137.209.in-addr.arpa. 5 NS ns-1016.awsdns-63.net.
192-255.135.137.209.in-addr.arpa. 5 NS ns-1989.awsdns-56.co.uk.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0