Apex domains cannot have CNAME records.
Question and response
Question
dig trestsko.net. NS
Answer
trestsko.net. 5 NS ns2.websupport.cz.
trestsko.net. 5 NS ns1.websupport.cz.
trestsko.net. 5 NS ns3.websupport.eu.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0