Apex domains cannot have CNAME records.
Question and response
Question
dig s3.sa-east-1.amazonaws.com. NS
Answer
s3.sa-east-1.amazonaws.com. 5 NS ns-623.awsdns-13.net.
s3.sa-east-1.amazonaws.com. 5 NS ns-1163.awsdns-17.org.
s3.sa-east-1.amazonaws.com. 5 NS ns-1803.awsdns-33.co.uk.
s3.sa-east-1.amazonaws.com. 5 NS ns-409.awsdns-51.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0