Apex domains cannot have CNAME records.
Question and response
Question
dig sa-east-1.compute.amazonaws.com. NS
Answer
sa-east-1.compute.amazonaws.com. 5 NS ns-1258.awsdns-29.org.
sa-east-1.compute.amazonaws.com. 5 NS ns-1681.awsdns-18.co.uk.
sa-east-1.compute.amazonaws.com. 5 NS ns-172.awsdns-21.com.
sa-east-1.compute.amazonaws.com. 5 NS ns-806.awsdns-36.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0