Apex domains cannot have CNAME records.
Question and response
Question
dig pt. NS
Answer
pt. 5 NS b.dns.pt.
pt. 5 NS h.dns.pt.
pt. 5 NS ns.dns.br.
pt. 5 NS e.dns.pt.
pt. 5 NS g.dns.pt.
pt. 5 NS a.dns.pt.
pt. 5 NS c.dns.pt.
pt. 5 NS ns2.nic.fr.
pt. 5 NS d.dns.pt.
Authority
The response has no authority records.
Additional
a.dns.pt. 117559 A 185.39.208.1
b.dns.pt. 172172 A 194.0.25.23
c.dns.pt. 172172 A 204.61.216.105
d.dns.pt. 117559 A 185.39.210.1
e.dns.pt. 117559 A 193.136.192.64
g.dns.pt. 117559 A 193.136.2.226
h.dns.pt. 117559 A 194.146.106.138
a.dns.pt. 117559 AAAA 2a04:6d80:0:0:0:0:0:1
b.dns.pt. 172172 AAAA 2001:678:20:0:0:0:0:23
c.dns.pt. 172172 AAAA 2001:500:14:6105:ad:0:0:1
d.dns.pt. 117559 AAAA 2a04:6d82:0:0:0:0:0:1
e.dns.pt. 117559 AAAA 2001:690:a00:4001:0:0:0:64
g.dns.pt. 117559 AAAA 2001:690:a80:4001:0:0:0:100
h.dns.pt. 117559 AAAA 2001:67c:1010:35:0:0:0:53
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0