Apex domains cannot have CNAME records.
Question and response
Question
dig sbb.rs. NS
Answer
sbb.rs. 4 NS dns2.sbb.rs.
sbb.rs. 4 NS dns1.sbb.rs.
sbb.rs. 4 NS dns4.sbb.rs.
sbb.rs. 4 NS dns3.sbb.rs.
Authority
The response has no authority records.
Additional
dns1.sbb.rs. 4 A 82.117.194.2
dns2.sbb.rs. 4 A 89.216.1.1
dns3.sbb.rs. 4 A 82.117.194.3
dns4.sbb.rs. 4 A 89.216.1.2
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0