Apex domains cannot have CNAME records.
Question and response
Question
dig ap-southeast-1.elb.amazonaws.com. NS
Answer
ap-southeast-1.elb.amazonaws.com. 5 NS ns-210.awsdns-26.com.
ap-southeast-1.elb.amazonaws.com. 5 NS ns-1125.awsdns-12.org.
ap-southeast-1.elb.amazonaws.com. 5 NS ns-785.awsdns-34.net.
ap-southeast-1.elb.amazonaws.com. 5 NS ns-1973.awsdns-54.co.uk.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0