Apex domains cannot have CNAME records.
Question and response
Question
dig s3-sa-east-1-w.amazonaws.com. NS
Answer
s3-sa-east-1-w.amazonaws.com. 5 NS ns-1514.awsdns-61.org.
s3-sa-east-1-w.amazonaws.com. 5 NS ns-612.awsdns-12.net.
s3-sa-east-1-w.amazonaws.com. 5 NS ns-231.awsdns-28.com.
s3-sa-east-1-w.amazonaws.com. 5 NS ns-1840.awsdns-38.co.uk.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0