Apex domains cannot have CNAME records.
Question and response
Question
dig securitynet.cz. NS
Answer
securitynet.cz. 5 NS ns3.hukot.cz.
securitynet.cz. 5 NS ns2.securitynet.cz.
securitynet.cz. 5 NS ns1.hukot.cz.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0