Apex domains cannot have CNAME records.
Question and response
Question
dig hosted-dkim.com. NS
Answer
hosted-dkim.com. 4 NS ns-1581.awsdns-05.co.uk.
hosted-dkim.com. 4 NS ns-57.awsdns-07.com.
hosted-dkim.com. 4 NS ns-549.awsdns-04.net.
hosted-dkim.com. 4 NS ns-1530.awsdns-63.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0