Apex domains cannot have CNAME records.
Question and response
Question
dig fei.stuba.sk. NS
Answer
fei.stuba.sk. 5 NS ns.cvt.stuba.sk.
fei.stuba.sk. 5 NS delta.fei.stuba.sk.
fei.stuba.sk. 5 NS ns.fei.stuba.sk.
Authority
The response has no authority records.
Additional
ns.cvt.stuba.sk. 5 A 147.175.1.11
ns.fei.stuba.sk. 5 A 147.175.111.15
ns.cvt.stuba.sk. 5 AAAA 2001:4118:10:100:0:0:0:11
ns.fei.stuba.sk. 5 AAAA 2001:4118:10:6f00:0:0:0:f
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0