Apex domains cannot have CNAME records.
Question and response
Question
dig eu-south-1.elb.amazonaws.com. NS
Answer
eu-south-1.elb.amazonaws.com. 5 NS ns-1675.awsdns-17.co.uk.
eu-south-1.elb.amazonaws.com. 5 NS ns-1100.awsdns-09.org.
eu-south-1.elb.amazonaws.com. 5 NS ns-190.awsdns-23.com.
eu-south-1.elb.amazonaws.com. 5 NS ns-940.awsdns-53.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0