Apex domains cannot have CNAME records.
Question and response
Question
dig hosted-by-discourse.com. NS
Answer
hosted-by-discourse.com. 5 NS ns-1250.awsdns-28.org.
hosted-by-discourse.com. 5 NS ns-104.awsdns-13.com.
hosted-by-discourse.com. 5 NS ns-717.awsdns-25.net.
hosted-by-discourse.com. 5 NS ns-2026.awsdns-61.co.uk.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0