Apex domains cannot have CNAME records.
Question and response
Question
dig EX-SIS.RU. NS
Answer
ex-sis.ru. 5 NS ns1.ex-sis.ru.
ex-sis.ru. 5 NS ns.ex-sis.ru.
Authority
The response has no authority records.
Additional
ns.EX-SIS.RU. 5 A 92.63.106.3
ns1.ex-sis.ru. 5 A 92.63.106.3
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0