Apex domains cannot have CNAME records.
Question and response
Question
dig REGISTER.COM. NS
Answer
REGISTER.COM. 5 NS heidi.ns.cloudflare.COM.
REGISTER.COM. 5 NS rex.ns.cloudflare.COM.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0