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