Apex domains cannot have CNAME records.
Question and response
Question
dig 128-191.236.188.210.in-addr.arpa. NS
Answer
128-191.236.188.210.in-addr.arpa. 5 NS ns-1484.awsdns-57.org.
128-191.236.188.210.in-addr.arpa. 5 NS ns-845.awsdns-41.net.
128-191.236.188.210.in-addr.arpa. 5 NS ns-472.awsdns-59.com.
128-191.236.188.210.in-addr.arpa. 5 NS ns-1738.awsdns-25.co.uk.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0