Apex domains cannot have CNAME records.
Question and response
Question
dig nethouse.cz. NS
Answer
nethouse.cz. 5 NS ns1.wia.cz.
nethouse.cz. 5 NS ns2.wia.cz.
Authority
The response has no authority records.
Additional
ns1.wia.cz. 5 A 80.250.5.90
ns2.wia.cz. 5 A 31.7.247.62
ns1.wia.cz. 5 AAAA 2a01:6400:2:1002:0:0:0:2
ns2.wia.cz. 5 AAAA 2a01:6400:2:1003:0:0:0:2
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0