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