Apex domains cannot have CNAME records.
Question and response
Question
dig s3-w.eu-west-1.amazonaws.com. NS
Answer
s3-w.eu-west-1.amazonaws.com. 5 NS ns-1760.awsdns-28.co.uk.
s3-w.eu-west-1.amazonaws.com. 5 NS ns-91.awsdns-11.com.
s3-w.eu-west-1.amazonaws.com. 5 NS ns-804.awsdns-36.net.
s3-w.eu-west-1.amazonaws.com. 5 NS ns-1474.awsdns-56.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0