Apex domains cannot have CNAME records.
Question and response
Question
dig sa-east-1.amazonaws.com. NS
Answer
sa-east-1.amazonaws.com. 5 NS ns-579.awsdns-08.net.
sa-east-1.amazonaws.com. 5 NS ns-1761.awsdns-28.co.uk.
sa-east-1.amazonaws.com. 5 NS ns-443.awsdns-55.com.
sa-east-1.amazonaws.com. 5 NS ns-1294.awsdns-33.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0