Apex domains cannot have CNAME records.
Question and response
Question
dig elb.ap-northeast-1.amazonaws.com. NS
Answer
elb.ap-northeast-1.amazonaws.com. 4 NS ns-1630.awsdns-11.co.uk.
elb.ap-northeast-1.amazonaws.com. 4 NS ns-1228.awsdns-25.org.
elb.ap-northeast-1.amazonaws.com. 4 NS ns-329.awsdns-41.com.
elb.ap-northeast-1.amazonaws.com. 4 NS ns-660.awsdns-18.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0