Apex domains cannot have CNAME records.
Question and response
Question
dig nic.cz. NS
Answer
nic.cz. 5 NS d.ns.nic.cz.
nic.cz. 5 NS a.ns.nic.cz.
nic.cz. 5 NS b.ns.nic.cz.
Authority
The response has no authority records.
Additional
a.ns.nic.cz. 109152 A 194.0.12.1
b.ns.nic.cz. 11328 A 194.0.13.1
d.ns.nic.cz. 143692 A 193.29.206.1
a.ns.nic.cz. 109152 AAAA 2001:678:f:0:0:0:0:1
b.ns.nic.cz. 11328 AAAA 2001:678:10:0:0:0:0:1
d.ns.nic.cz. 143692 AAAA 2001:678:1:0:0:0:0:1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0