Apex domains cannot have CNAME records.
Question and response
Question
dig wmx.slovanet.sk. NS
Answer
wmx.slovanet.sk. 1 NS ns.slovanet.net.
wmx.slovanet.sk. 1 NS ns1.slovanet.net.
Authority
The response has no authority records.
Additional
ns.slovanet.net. 1 A 195.80.171.4
ns1.slovanet.net. 1 A 195.28.64.119
ns.slovanet.net. 1 AAAA 2a02:dd8:ff:201:0:0:c350:ab04
ns1.slovanet.net. 1 AAAA 2a02:dd8:ff:301:0:0:c31c:4077
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0