Apex domains cannot have CNAME records.
Question and response
Question
dig auth.acme-dns.io. NS
Answer
auth.acme-dns.io. 5 NS ns.auth.acme-dns.io.
Authority
The response has no authority records.
Additional
ns.auth.acme-dns.io. 5 A 49.12.60.200
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0