Apex domains cannot have CNAME records.
Question and response
Question
dig impsat.net. NS
Answer
impsat.net. 4 NS ns1.us.impsat.net.
impsat.net. 4 NS ns2.us.impsat.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0