Apex domains cannot have CNAME records.
Question and response
Question
dig sa-east-1.elb.amazonaws.com. NS
Answer
sa-east-1.elb.amazonaws.com. 5 NS ns-2034.awsdns-62.co.uk.
sa-east-1.elb.amazonaws.com. 5 NS ns-1276.awsdns-31.org.
sa-east-1.elb.amazonaws.com. 5 NS ns-291.awsdns-36.com.
sa-east-1.elb.amazonaws.com. 5 NS ns-632.awsdns-15.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0