Apex domains cannot have CNAME records.
Question and response
Question
dig corp1.discourse.cloud. NS
Answer
corp1.discourse.cloud. 1 NS ns3.discourse.cloud.
corp1.discourse.cloud. 1 NS ns1a.corp1.discourse.cloud.
corp1.discourse.cloud. 1 NS ns1b.corp1.discourse.cloud.
corp1.discourse.cloud. 1 NS ns2.discourse.cloud.
corp1.discourse.cloud. 1 NS ns1.discourse.cloud.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0