Apex domains cannot have CNAME records.
Question and response
Question
dig elb.eu-central-1.amazonaws.com. NS
Answer
elb.eu-central-1.amazonaws.com. 5 NS ns-1390.awsdns-45.org.
elb.eu-central-1.amazonaws.com. 5 NS ns-1853.awsdns-39.co.uk.
elb.eu-central-1.amazonaws.com. 5 NS ns-271.awsdns-33.com.
elb.eu-central-1.amazonaws.com. 5 NS ns-777.awsdns-33.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0