Apex domains cannot have CNAME records.
Question and response
Question
dig dkim.amazonses.com. NS
Answer
dkim.amazonses.com. 5 NS ns-1875.awsdns-42.co.uk.
dkim.amazonses.com. 5 NS ns-1238.awsdns-26.org.
dkim.amazonses.com. 5 NS ns-386.awsdns-48.com.
dkim.amazonses.com. 5 NS ns-525.awsdns-01.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0