Apex domains cannot have CNAME records.
Question and response
Question
dig dns.pt. NS
Answer
dns.pt. 5 NS b.dns.pt.
dns.pt. 5 NS nsp.dnsnode.net.
dns.pt. 5 NS c.dns.pt.
dns.pt. 5 NS dns01.dns.pt.
dns.pt. 5 NS dns02.dns.pt.
dns.pt. 5 NS europe1.dnsnode.net.
Authority
The response has no authority records.
Additional
b.dns.pt. 171850 A 194.0.25.23
c.dns.pt. 171850 A 204.61.216.105
b.dns.pt. 171850 AAAA 2001:678:20:0:0:0:0:23
c.dns.pt. 171850 AAAA 2001:500:14:6105:ad:0:0:1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0