Apex domains cannot have CNAME records.
Question and response
Question
dig pd.uniza.sk. NS
Answer
pd.uniza.sk. 4 NS ns2.pd.uniza.sk.
pd.uniza.sk. 4 NS ns1.pd.uniza.sk.
Authority
The response has no authority records.
Additional
ns1.pd.uniza.sk. 4 A 194.160.135.33
ns2.pd.uniza.sk. 4 A 194.160.135.36
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0