Apex domains cannot have CNAME records.
Question and response
Question
dig moderndns.net. NS
Answer
moderndns.net. 5 NS ns-523.awsdns-01.net.
moderndns.net. 5 NS ns-766.awsdns-31.net.
moderndns.net. 5 NS ns-1377.awsdns-44.org.
moderndns.net. 5 NS ns-1982.awsdns-55.co.uk.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0