Apex domains cannot have CNAME records.
Question and response
Question
dig ap-south-1.elb.amazonaws.com. NS
Answer
ap-south-1.elb.amazonaws.com. 5 NS ns-1568.awsdns-04.co.uk.
ap-south-1.elb.amazonaws.com. 5 NS ns-1498.awsdns-59.org.
ap-south-1.elb.amazonaws.com. 5 NS ns-329.awsdns-41.com.
ap-south-1.elb.amazonaws.com. 5 NS ns-836.awsdns-40.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0