Apex domains cannot have CNAME records.
Question and response
Question
dig cz.net. NS
Answer
cz.net. 5 NS nic.eunet.cz.
cz.net. 5 NS ns.eunet.cz.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0