Apex domains cannot have CNAME records.
Question and response
Question
dig security.cloudflare-dns.com. NS
Answer
security.cloudflare-dns.com. 4 NS meera.ns.cloudflare.com.
security.cloudflare-dns.com. 4 NS bart.ns.cloudflare.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0