Apex domains cannot have CNAME records.
Question and response
Question
dig ns.ao. NS
Answer
ns.ao. 5 NS dns52.cloudns.net.
ns.ao. 5 NS dns53.cloudns.net.
ns.ao. 5 NS dns54.cloudns.net.
ns.ao. 5 NS dns51.cloudns.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0