Apex domains cannot have CNAME records.
Question and response
Question
dig elb.eu-west-1.amazonaws.com. NS
Answer
elb.eu-west-1.amazonaws.com. 5 NS ns-1566.awsdns-03.co.uk.
elb.eu-west-1.amazonaws.com. 5 NS ns-1243.awsdns-27.org.
elb.eu-west-1.amazonaws.com. 5 NS ns-272.awsdns-34.com.
elb.eu-west-1.amazonaws.com. 5 NS ns-638.awsdns-15.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0