Apex domains cannot have CNAME records.
Question and response
Question
dig ig.cas.cz. NS
Answer
ig.cas.cz. 2 NS gandalf.ig.cas.cz.
ig.cas.cz. 2 NS visla.utia.cas.cz.
Authority
The response has no authority records.
Additional
visla.utia.cas.cz. 2 A 147.231.12.1
gandalf.ig.cas.cz. 2 A 147.231.72.50
visla.utia.cas.cz. 2 AAAA 2001:718:401:1:0:0:0:1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0