Apex domains cannot have CNAME records.
Question and response
Question
dig a1.hr. NS
Answer
a1.hr. 5 NS dnsb.a1.hr.
a1.hr. 5 NS dnsa.a1.hr.
Authority
The response has no authority records.
Additional
dnsa.a1.hr. 5 A 212.91.97.5
dnsb.a1.hr. 5 A 212.91.97.6
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0