Apex domains cannot have CNAME records.
Question and response
Question
dig fnhk.cz. NS
Answer
fnhk.cz. 3 NS slimak.fnhk.cz.
fnhk.cz. 3 NS dns2.fnhk.cz.
fnhk.cz. 3 NS ns.hknet.cz.
Authority
The response has no authority records.
Additional
ns.hknet.cz. 3 A 195.113.115.171
dns2.fnhk.cz. 3 A 89.248.244.34
slimak.fnhk.cz. 3 A 90.181.244.18
slimak.fnhk.cz. 3 A 195.113.123.85
slimak.fnhk.cz. 3 AAAA 2001:718:1200:6:195:113:123:85
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0