Apex domains cannot have CNAME records.
Question and response
Question
dig stable.sk. NS
Answer
stable.sk. 5 NS ns.stable.sk.
stable.sk. 5 NS ns.stable.cz.
Authority
The response has no authority records.
Additional
ns.stable.cz. 5 A 217.31.48.201
ns.stable.sk. 5 A 185.65.223.35
ns.stable.cz. 5 AAAA 2001:1ab0:7e1e:2:0:0:0:5
ns.stable.sk. 5 AAAA 2a00:1e40:a:53:0:0:0:5
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0