Apex domains cannot have CNAME records.
Question and response
Question
dig optinet.hr. NS
Answer
optinet.hr. 5 NS bjesomar.srce.hr.
optinet.hr. 5 NS dns2.optima-telekom.hr.
optinet.hr. 5 NS dns1.optima-telekom.hr.
Authority
The response has no authority records.
Additional
dns1.optima-telekom.hr. 5 A 85.114.32.5
dns2.optima-telekom.hr. 5 A 85.114.32.6
bjesomar.srce.hr. 5 A 161.53.2.70
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0