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