Apex domains cannot have CNAME records.
Question and response
Question
dig ap-south-1.compute.amazonaws.com. NS
Answer
ap-south-1.compute.amazonaws.com. 5 NS ns-783.awsdns-33.net.
ap-south-1.compute.amazonaws.com. 5 NS ns-1469.awsdns-55.org.
ap-south-1.compute.amazonaws.com. 5 NS ns-1992.awsdns-57.co.uk.
ap-south-1.compute.amazonaws.com. 5 NS ns-293.awsdns-36.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0