Apex domains cannot have CNAME records.
Question and response
Question
dig nextdns.io. NS
Answer
nextdns.io. 4 NS dawn.ns.cloudflare.com.
nextdns.io. 4 NS lee.ns.cloudflare.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0