Apex domains cannot have CNAME records.
Question and response
Question
dig pt.lu. NS
Answer
pt.lu. 5 NS ns3.pt.lu.
pt.lu. 5 NS ns2.pt.lu.
pt.lu. 5 NS ns1.pt.lu.
Authority
The response has no authority records.
Additional
ns1.pt.lu. 5 A 194.154.192.1
ns2.pt.lu. 5 A 195.46.252.2
ns3.pt.lu. 5 A 80.90.63.90
ns1.pt.lu. 5 AAAA 2001:7e8:8800:0:0:0:0:b0
ns2.pt.lu. 5 AAAA 2001:7e8:8800:0:0:0:0:c0
ns3.pt.lu. 5 AAAA 2001:1610:2:1:0:0:0:90
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0