Apex domains cannot have CNAME records.
Question and response
Question
dig 4system.sk. NS
Answer
4system.sk. 4 NS ns1.websupport.sk.
4system.sk. 4 NS ns3.websupport.sk.
4system.sk. 4 NS ns2.websupport.sk.
Authority
The response has no authority records.
Additional
ns1.websupport.sk. 4 A 45.10.97.11
ns2.websupport.sk. 4 A 185.71.159.12
ns3.websupport.sk. 4 A 45.10.97.13
ns1.websupport.sk. 4 AAAA 2a10:bbc0:0:0:0:0:0:11
ns2.websupport.sk. 4 AAAA 2a02:250:3:0:0:0:0:12
ns3.websupport.sk. 4 AAAA 2a10:bbc0:0:0:0:0:0:13
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0