Apex domains cannot have CNAME records.
Question and response
Question
dig netbox.cz. NS
Answer
netbox.cz. 5 NS ns2.netbox.cz.
netbox.cz. 5 NS ns1.netbox.cz.
Authority
The response has no authority records.
Additional
ns1.netbox.cz. 5 A 83.240.0.186
ns2.netbox.cz. 5 A 83.240.0.187
ns1.netbox.cz. 5 AAAA 2001:4cc8:0:3:0:0:0:1111
ns2.netbox.cz. 5 AAAA 2001:4cc8:0:3:0:0:0:8888
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0