Apex domains cannot have CNAME records.
Question and response
Question
dig pranet.cz. NS
Answer
pranet.cz. 5 NS ns2.blueboard.cz.
pranet.cz. 5 NS shinzon.blueboard.cz.
Authority
The response has no authority records.
Additional
ns2.blueboard.cz. 4 AAAA 2a01:4f8:13b:8d4:0:0:0:2
shinzon.blueboard.cz. 4 AAAA 2a00:1ed0:45:0:0:0:0:b
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0