Apex domains cannot have CNAME records.
Question and response
Question
dig oxit.cz. NS
Answer
oxit.cz. 5 NS shinzon.blueboard.cz.
oxit.cz. 5 NS ns2.blueboard.cz.
Authority
The response has no authority records.
Additional
ns2.blueboard.cz. 5 A 195.201.57.122
shinzon.blueboard.cz. 5 A 178.251.187.7
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0