Apex domains cannot have CNAME records.
Question and response
Question
dig s3preview.com. NS
Answer
s3preview.com. 5 NS ns-649.awsdns-17.net.
s3preview.com. 5 NS ns-284.awsdns-35.com.
s3preview.com. 5 NS ns-1855.awsdns-39.co.uk.
s3preview.com. 5 NS ns-667.awsdns-19.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0