Apex domains cannot have CNAME records.
Question and response
Question
dig webcerta.com.br. NS
Answer
webcerta.com.br. 5 NS ns1.webcerta.com.br.
Authority
The response has no authority records.
Additional
ns1.webcerta.com.br. 5 A 51.222.47.120
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0