Apex domains cannot have CNAME records.
Question and response
Question
dig uniza.sk. NS
Answer
uniza.sk. 5 NS proxy.uniza.sk.
uniza.sk. 5 NS sun.uakom.sk.
uniza.sk. 5 NS nic.uniza.sk.
Authority
The response has no authority records.
Additional
nic.uniza.sk. 5 A 158.193.48.33
sun.uakom.sk. 5 A 192.108.131.11
proxy.uniza.sk. 5 A 158.193.1.10
nic.uniza.sk. 5 AAAA 2001:4118:300:48:0:0:0:33
sun.uakom.sk. 5 AAAA 2001:4118:100:1:0:0:0:11
proxy.uniza.sk. 5 AAAA 2001:4118:300:1:0:0:0:10
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0