Apex domains cannot have CNAME records.
Question and response
Question
dig edu.sk. NS
Answer
edu.sk. 5 NS ns2.telecom.sk.
edu.sk. 5 NS ns.telecom.sk.
Authority
The response has no authority records.
Additional
ns.telecom.sk. 5 A 213.81.153.153
ns2.telecom.sk. 5 A 213.81.170.170
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0