Apex domains cannot have CNAME records.
Question and response
Question
dig acme-dns.io. NS
Answer
acme-dns.io. 5 NS pablo.ns.cloudflare.com.
acme-dns.io. 5 NS bonnie.ns.cloudflare.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0