Apex domains cannot have CNAME records.
Question and response
Question
dig mundivox.com. NS
Answer
mundivox.com. 1 NS ns-rj-2.mundivox.com.
mundivox.com. 1 NS ns-rj-1.mundivox.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0