Apex domains cannot have CNAME records.
Question and response
Question
dig eu-south-1.amazonses.com. NS
Answer
eu-south-1.amazonses.com. 3 NS ns-999.awsdns-60.net.
eu-south-1.amazonses.com. 3 NS ns-1714.awsdns-22.co.uk.
eu-south-1.amazonses.com. 3 NS ns-495.awsdns-61.com.
eu-south-1.amazonses.com. 3 NS ns-1283.awsdns-32.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0