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