Apex domains cannot have CNAME records.
Question and response
Question
dig cm-castroverde.pt. NS
Answer
cm-castroverde.pt. 5 NS dns2.semcabo.pt.
cm-castroverde.pt. 5 NS dns1.semcabo.pt.
Authority
The response has no authority records.
Additional
dns1.semcabo.pt. 5 A 188.93.235.1
dns2.semcabo.pt. 5 A 188.93.235.2
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0