Apex domains cannot have CNAME records.
Question and response
Question
dig nfx.cz. NS
Answer
nfx.cz. 2 NS nss.nfx.cz.
nfx.cz. 2 NS ns.nfx.cz.
Authority
The response has no authority records.
Additional
ns.nfx.cz. 2 A 81.201.48.178
nss.nfx.cz. 2 A 109.205.73.191
ns.nfx.cz. 2 AAAA 2a01:490:0:2:0:0:0:178
nss.nfx.cz. 2 AAAA 2a00:1238:0:3531:0:0:0:2
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0