Apex domains cannot have CNAME records.
Question and response
Question
dig elb.us-west-2.amazonaws.com. NS
Answer
elb.us-west-2.amazonaws.com. 5 NS ns-424.awsdns-53.com.
elb.us-west-2.amazonaws.com. 5 NS ns-1283.awsdns-32.org.
elb.us-west-2.amazonaws.com. 5 NS ns-748.awsdns-29.net.
elb.us-west-2.amazonaws.com. 5 NS ns-1870.awsdns-41.co.uk.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0