Apex domains cannot have CNAME records.
Question and response
Question
dig ao. NS
Answer
ao. 5 NS etld-1.anycast.net.
ao. 5 NS ao02.dns.pt.
ao. 5 NS a.ns.ao.
ao. 5 NS ao-e.ns.nic.cz.
Authority
The response has no authority records.
Additional
a.ns.ao. 19626 A 206.51.254.4
a.ns.ao. 19626 AAAA 2620:171:804:ad2:0:0:0:4
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0