Apex domains cannot have CNAME records.
Question and response
Question
dig previsao-sgfp.pt. NS
Answer
previsao-sgfp.pt. 4 NS ns2.ptempresas.pt.
previsao-sgfp.pt. 4 NS ns10.ptempresas.pt.
previsao-sgfp.pt. 4 NS ns1.ptempresas.pt.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0