Apex domains cannot have CNAME records.
Question and response
Question
dig impsat.net.br. NS
Answer
impsat.net.br. 5 NS ns-152-c.gandi.net.
impsat.net.br. 5 NS ns-182-a.gandi.net.
impsat.net.br. 5 NS ns-99-b.gandi.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0