Apex domains cannot have CNAME records.
Question and response
Question
dig bg.AC.RS. NS
Answer
bg.ac.rs. 5 NS ns.rcub.bg.ac.rs.
bg.ac.rs. 5 NS gaea.rcub.bg.ac.rs.
bg.ac.rs. 5 NS ban.junis.ni.ac.rs.
bg.ac.rs. 5 NS ns1.uns.ac.rs.
bg.ac.rs. 5 NS ns.etf.bg.ac.rs.
bg.ac.rs. 5 NS ns.unic.kg.ac.rs.
Authority
The response has no authority records.
Additional
ns.etf.bg.ac.rs. 5 A 147.91.8.6
ns.rcub.bg.ac.rs. 5 A 147.91.1.5
ns.unic.kg.ac.rs. 5 A 147.91.209.2
ban.junis.ni.ac.rs. 5 A 160.99.1.1
ns1.uns.ac.rs. 5 A 147.91.173.4
gaea.rcub.bg.ac.rs. 5 A 147.91.1.7
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0