Apex domains cannot have CNAME records.
Question and response
Question
dig casablanca.cz. NS
Answer
casablanca.cz. 5 NS slave.casablanca.cz.
casablanca.cz. 5 NS ns2.casablanca.cz.
casablanca.cz. 5 NS ns1.casablanca.cz.
Authority
The response has no authority records.
Additional
ns1.casablanca.cz. 5 A 217.11.224.3
ns2.casablanca.cz. 5 A 217.11.224.4
slave.casablanca.cz. 5 A 193.165.246.74
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0