Apex domains cannot have CNAME records.
Question and response
Question
dig hknet.cz. NS
Answer
hknet.cz. 3 NS ns2.hknet.cz.
hknet.cz. 3 NS ns.hknet.cz.
hknet.cz. 3 NS nsa.ces.net.
Authority
The response has no authority records.
Additional
ns.hknet.cz. 3 A 195.113.115.171
ns2.hknet.cz. 3 A 195.113.115.174
nsa.ces.net. 3 A 195.113.144.205
nsa.ces.net. 3 AAAA 2001:718:1:1:0:0:144:205
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0