Apex domains cannot have CNAME records.
Question and response
Question
dig meo.pt. NS
Answer
meo.pt. 5 NS ns2.telepac.pt.
meo.pt. 5 NS ns4.telepac.pt.
meo.pt. 5 NS ns3.telepac.pt.
meo.pt. 5 NS coltrane.telepac.pt.
Authority
The response has no authority records.
Additional
ns2.telepac.pt. 5 A 194.65.3.21
ns4.telepac.pt. 5 A 194.65.3.103
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0