Apex domains cannot have CNAME records.
Question and response
Question
dig mammothmpegs.com. NS
Answer
mammothmpegs.com. 5 NS ns-1719.awsdns-22.co.uk.
mammothmpegs.com. 5 NS ns-731.awsdns-27.net.
mammothmpegs.com. 5 NS ns-621.awsdns-13.net.
mammothmpegs.com. 5 NS ns-2007.awsdns-58.co.uk.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0