Apex domains cannot have CNAME records.
Question and response
Question
dig sofia.sk. NS
Answer
sofia.sk. 5 NS helium.ns.hetzner.com.
sofia.sk. 5 NS hydrogen.ns.hetzner.com.
sofia.sk. 5 NS ns2.forma.sk.
sofia.sk. 5 NS ns1.forma.sk.
sofia.sk. 5 NS ns.forma.sk.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0