Apex domains cannot have CNAME records.
Question and response
Question
dig elb.us-east-1.amazonaws.com. NS
Answer
elb.us-east-1.amazonaws.com. 5 NS ns-1826.awsdns-36.co.uk.
elb.us-east-1.amazonaws.com. 5 NS ns-844.awsdns-41.net.
elb.us-east-1.amazonaws.com. 5 NS ns-281.awsdns-35.com.
elb.us-east-1.amazonaws.com. 5 NS ns-1286.awsdns-32.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0