Apex domains cannot have CNAME records.
Question and response
Question
dig 48-63.218.36.138.in-addr.arpa. NS
Answer
48-63.218.36.138.in-addr.arpa. 5 NS ns-954.awsdns-55.net.
48-63.218.36.138.in-addr.arpa. 5 NS ns-1587.awsdns-06.co.uk.
48-63.218.36.138.in-addr.arpa. 5 NS ns-60.awsdns-07.com.
48-63.218.36.138.in-addr.arpa. 5 NS ns-1128.awsdns-13.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0