Apex domains cannot have CNAME records.
Question and response
Question
dig sea1.discourse.cloud. NS
Answer
sea1.discourse.cloud. 5 NS ns1b.sea1.discourse.cloud.
sea1.discourse.cloud. 5 NS ns2.discourse.cloud.
sea1.discourse.cloud. 5 NS ns3.discourse.cloud.
sea1.discourse.cloud. 5 NS ns1a.sea1.discourse.cloud.
sea1.discourse.cloud. 5 NS ns1.discourse.cloud.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0