Apex domains cannot have CNAME records.
Question and response
Question
dig discourse.cloud. NS
Answer
discourse.cloud. 4 NS ns-1012.awsdns-62.net.
discourse.cloud. 4 NS ns-1963.awsdns-53.co.uk.
discourse.cloud. 4 NS ns-410.awsdns-51.com.
discourse.cloud. 4 NS ns-1094.awsdns-08.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0