Apex domains cannot have CNAME records.
Question and response
Question
dig s3.eu-west-1.amazonaws.com. NS
Answer
s3.eu-west-1.amazonaws.com. 5 NS ns-1262.awsdns-29.org.
s3.eu-west-1.amazonaws.com. 5 NS ns-781.awsdns-33.net.
s3.eu-west-1.amazonaws.com. 5 NS ns-96.awsdns-12.com.
s3.eu-west-1.amazonaws.com. 5 NS ns-1925.awsdns-48.co.uk.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0