Apex domains cannot have CNAME records.
Question and response
Question
dig elb.ap-southeast-1.amazonaws.com. NS
Answer
elb.ap-southeast-1.amazonaws.com. 5 NS ns-1873.awsdns-42.co.uk.
elb.ap-southeast-1.amazonaws.com. 5 NS ns-928.awsdns-52.net.
elb.ap-southeast-1.amazonaws.com. 5 NS ns-448.awsdns-56.com.
elb.ap-southeast-1.amazonaws.com. 5 NS ns-1082.awsdns-07.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0