Apex domains cannot have CNAME records.
Question and response
Question
dig servers.ru. NS
Answer
servers.ru. 1 NS ns1.he.net.
servers.ru. 1 NS ns2.srvdns.ru.
servers.ru. 1 NS ns2.he.net.
servers.ru. 1 NS ns1.srvdns.ru.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0