Apex domains cannot have CNAME records.
Question and response
Question
dig corpnet.cz. NS
Answer
corpnet.cz. 5 NS ns.corpnet.cz.
corpnet.cz. 5 NS ns2.skynet.cz.
corpnet.cz. 5 NS ns2.vitkovice.cz.
Authority
The response has no authority records.
Additional
ns.corpnet.cz. 5 A 176.67.224.50
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0