Apex domains cannot have CNAME records.
Question and response
Question
dig s3-w.sa-east-1.amazonaws.com. NS
Answer
s3-w.sa-east-1.amazonaws.com. 5 NS ns-1530.awsdns-63.org.
s3-w.sa-east-1.amazonaws.com. 5 NS ns-593.awsdns-10.net.
s3-w.sa-east-1.amazonaws.com. 5 NS ns-297.awsdns-37.com.
s3-w.sa-east-1.amazonaws.com. 5 NS ns-1678.awsdns-17.co.uk.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0