Apex domains cannot have CNAME records.
Question and response
Question
dig hostingzdarma.cz. NS
Answer
hostingzdarma.cz. 4 NS ns2.hostingzdarma.cz.
hostingzdarma.cz. 4 NS ns1.hostingzdarma.cz.
hostingzdarma.cz. 4 NS ns3.hosting-zdarma.net.
Authority
The response has no authority records.
Additional
ns1.hostingzdarma.cz. 4 A 185.65.223.35
ns2.hostingzdarma.cz. 4 A 217.31.48.201
ns3.hosting-zdarma.net. 4 A 178.62.60.201
ns1.hostingzdarma.cz. 4 AAAA 2a00:1e40:a:53:0:0:0:5
ns2.hostingzdarma.cz. 4 AAAA 2001:1ab0:7e1e:2:0:0:0:5
ns3.hosting-zdarma.net. 4 AAAA 2a03:b0c0:1:d0:0:0:1090:2001
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0