Apex domains cannot have CNAME records.
Question and response
Question
dig s3.ca-central-1.amazonaws.com. NS
Answer
s3.ca-central-1.amazonaws.com. 5 NS ns-1595.awsdns-07.co.uk.
s3.ca-central-1.amazonaws.com. 5 NS ns-936.awsdns-53.net.
s3.ca-central-1.amazonaws.com. 5 NS ns-427.awsdns-53.com.
s3.ca-central-1.amazonaws.com. 5 NS ns-1327.awsdns-37.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0