Apex domains cannot have CNAME records.
Question and response
Question
dig rt.ru. NS
Answer
rt.ru. 5 NS ns8-cloud.nic.ru.
rt.ru. 5 NS ns4-cloud.nic.ru.
rt.ru. 5 NS ns8-l2.nic.ru.
rt.ru. 5 NS ns1.rostelecom.ru.
rt.ru. 5 NS ns4-l2.nic.ru.
rt.ru. 5 NS ns2.rostelecom.ru.
Authority
The response has no authority records.
Additional
ns1.rostelecom.ru. 5 A 87.226.162.62
ns2.rostelecom.ru. 5 A 87.226.159.65
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0