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