Apex domains cannot have CNAME records.
Question and response
Question
dig cz. NS
Answer
cz. 5 NS d.ns.nic.cz.
cz. 5 NS b.ns.nic.cz.
cz. 5 NS a.ns.nic.cz.
cz. 5 NS c.ns.nic.cz.
Authority
The response has no authority records.
Additional
a.ns.nic.cz. 56767 A 194.0.12.1
b.ns.nic.cz. 59387 A 194.0.13.1
c.ns.nic.cz. 60218 A 194.0.14.1
d.ns.nic.cz. 57284 A 193.29.206.1
a.ns.nic.cz. 56767 AAAA 2001:678:f:0:0:0:0:1
b.ns.nic.cz. 59387 AAAA 2001:678:10:0:0:0:0:1
c.ns.nic.cz. 60218 AAAA 2001:678:11:0:0:0:0:1
d.ns.nic.cz. 57284 AAAA 2001:678:1:0:0:0:0:1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0