Apex domains cannot have CNAME records.
Question and response
Question
dig rDNS.SiEL.si. NS
Answer
rDNS.SiEL.si. 5 NS dns2.SiEL.si.
rDNS.SiEL.si. 5 NS dns1.SiEL.si.
rDNS.SiEL.si. 5 NS dns3.SiEL.si.
Authority
The response has no authority records.
Additional
dns1.SiEL.si. 5 A 46.19.12.12
dns2.siel.si. 5 A 46.19.13.13
dns3.siel.si. 5 A 217.61.252.53
dns1.SiEL.si. 5 AAAA 2a02:28b0:53:0:0:0:0:53
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0