Apex domains cannot have CNAME records.
Question and response
Question
dig sbone.cz. NS
Answer
sbone.cz. 5 NS ns.inecnet.cz.
sbone.cz. 5 NS ns2.inecnet.cz.
Authority
The response has no authority records.
Additional
ns.inecnet.cz. 5 A 212.47.0.10
ns2.inecnet.cz. 5 A 195.39.37.138
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0