Apex domains cannot have CNAME records.
Question and response
Question
dig dmz.dns.pt. NS
Answer
dmz.dns.pt. 4 NS dns01.dns.pt.
dmz.dns.pt. 4 NS dns02.dns.pt.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0