Apex domains cannot have CNAME records.
Question and response
Question
dig gw. NS
Answer
gw. 5 NS gw03.dns.pt.
gw. 5 NS gw01.dns.pt.
gw. 5 NS h.dns.pt.
Authority
The response has no authority records.
Additional
h.dns.pt. 166566 A 194.146.106.138
h.dns.pt. 166566 AAAA 2001:67c:1010:35:0:0:0:53
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0