Apex domains cannot have CNAME records.
Question and response
Question
dig rcub.bg.AC.RS. NS
Answer
rcub.bg.ac.rs. 5 NS gaea.rcub.bg.ac.rs.
rcub.bg.ac.rs. 5 NS ns.rcub.bg.ac.rs.
rcub.bg.ac.rs. 5 NS ns.etf.bg.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
gaea.rcub.bg.ac.rs. 5 A 147.91.1.7
ns.rcub.bg.ac.rs. 5 AAAA 2001:4170:0:1:0:0:0:5
gaea.rcub.bg.ac.rs. 5 AAAA 2001:4170:0:1:0:0:0:7
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0