Apex domains cannot have CNAME records.
Question and response
Question
dig 64-79.218.36.138.in-addr.arpa. NS
Answer
64-79.218.36.138.in-addr.arpa. 4 NS ns-1234.awsdns-26.org.
64-79.218.36.138.in-addr.arpa. 4 NS ns-1701.awsdns-20.co.uk.
64-79.218.36.138.in-addr.arpa. 4 NS ns-317.awsdns-39.com.
64-79.218.36.138.in-addr.arpa. 4 NS ns-705.awsdns-24.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0