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