Apex domains cannot have CNAME records.
Question and response
Question
dig heureka.cz. NS
Answer
heureka.cz. 5 NS rajeev.ns.cloudflare.com.
heureka.cz. 5 NS marjory.ns.cloudflare.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0