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-1042.awsdns-02.org.
s3-sa-east-1.amazonaws.com. 5 NS ns-182.awsdns-22.com.
s3-sa-east-1.amazonaws.com. 5 NS ns-945.awsdns-54.net.
s3-sa-east-1.amazonaws.com. 5 NS ns-1878.awsdns-42.co.uk.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0