Apex domains cannot have CNAME records.
Question and response
Question
dig 221.25.171.in-addr.arpa. NS
Answer
221.25.171.in-addr.arpa. 4 NS ns-1916.awsdns-47.co.uk.
221.25.171.in-addr.arpa. 4 NS ns-1464.awsdns-55.org.
221.25.171.in-addr.arpa. 4 NS ns-298.awsdns-37.com.
221.25.171.in-addr.arpa. 4 NS ns-707.awsdns-24.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0