Apex domains cannot have CNAME records.
Question and response
Question
dig co.sapo.pt. NS
Answer
co.sapo.pt. 5 NS ns2.sapo.pt.
co.sapo.pt. 5 NS ns.sapo.pt.
Authority
The response has no authority records.
Additional
ns.sapo.pt. 5 A 212.55.154.202
ns2.sapo.pt. 5 A 212.55.154.194
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0