Apex domains cannot have CNAME records.
Question and response
Question
dig secureinternet.cz. NS
Answer
secureinternet.cz. 4 NS ns2.gts.cz.
secureinternet.cz. 4 NS ns1.gts.cz.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0