Apex domains cannot have CNAME records.
Question and response
Question
dig sky.cz. NS
Answer
sky.cz. 5 NS ns3.poda.cz.
sky.cz. 5 NS ns1.poda.cz.
sky.cz. 5 NS ns2.poda.cz.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0