Apex domains cannot have CNAME records.
Question and response
Question
dig fei.tuke.sk. NS
Answer
fei.tuke.sk. 5 NS oko.fei.tuke.sk.
fei.tuke.sk. 5 NS zub.fei.tuke.sk.
fei.tuke.sk. 5 NS ns.vychod.net.
Authority
The response has no authority records.
Additional
ns.vychod.net. 5 A 84.245.77.66
oko.fei.tuke.sk. 5 A 147.232.40.1
zub.fei.tuke.sk. 5 A 147.232.40.7
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0