Apex domains cannot have CNAME records.
Question and response
Question
dig s3-r-w.sa-east-1.amazonaws.com. NS
Answer
s3-r-w.sa-east-1.amazonaws.com. 5 NS ns-1902.awsdns-45.co.uk.
s3-r-w.sa-east-1.amazonaws.com. 5 NS ns-85.awsdns-10.com.
s3-r-w.sa-east-1.amazonaws.com. 5 NS ns-933.awsdns-52.net.
s3-r-w.sa-east-1.amazonaws.com. 5 NS ns-1249.awsdns-28.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0