Apex domains cannot have CNAME records.
Question and response
Question
dig zetta.io. NS
Answer
zetta.io. 5 NS ns1.zetta.io.
zetta.io. 5 NS ns2.zetta.io.
Authority
The response has no authority records.
Additional
ns1.zetta.io. 5 A 109.239.235.42
ns2.zetta.io. 5 A 185.56.185.64
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0