Apex domains cannot have CNAME records.
Question and response
Question
dig fxo.us. NS
Answer
fxo.us. 5 NS ns-608.awsdns-12.net.
fxo.us. 5 NS ns-1391.awsdns-45.org.
fxo.us. 5 NS ns-1697.awsdns-20.co.uk.
fxo.us. 5 NS ns-482.awsdns-60.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0