Apex domains cannot have CNAME records.
Question and response
Question
dig optimus.si. NS
Answer
optimus.si. 5 NS ns1.optimus.si.
optimus.si. 5 NS ns5.isg.si.
optimus.si. 5 NS ns6.isg.si.
optimus.si. 5 NS cns.isg.si.
optimus.si. 5 NS ns2.amis.net.
optimus.si. 5 NS ns3.amis.net.
Authority
The response has no authority records.
Additional
cns.isg.si. 5 A 91.223.182.11
ns1.optimus.si. 5 A 91.223.182.124
ns2.amis.net. 5 A 80.95.234.33
ns3.amis.net. 5 A 212.18.32.142
ns5.isg.si. 5 A 193.9.21.12
ns6.isg.si. 5 A 84.255.203.52
ns2.amis.net. 5 AAAA 2a00:1a20:40:2:0:0:0:12
ns3.amis.net. 5 AAAA 2001:15c0:ffff:12:0:0:0:140
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0