Apex domains cannot have CNAME records.
Question and response
Question
dig rexum.space. NS
Answer
rexum.space. 5 NS elma.ns.cloudflare.com.
rexum.space. 5 NS rob.ns.cloudflare.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0