Apex domains cannot have CNAME records.
Question and response
Question
dig my.lu. NS
Answer
my.lu. 5 NS ns1.restena.lu.
my.lu. 5 NS ns3.restena.lu.
my.lu. 5 NS nsp.restena.lu.
my.lu. 5 NS ns2.restena.lu.
my.lu. 5 NS nse.restena.lu.
Authority
The response has no authority records.
Additional
ns1.restena.lu. 5 A 158.64.1.23
ns2.restena.lu. 5 A 91.92.119.210
ns3.restena.lu. 5 A 158.64.12.45
nse.restena.lu. 5 A 194.58.196.32
nsp.restena.lu. 5 A 194.58.198.32
ns1.restena.lu. 5 AAAA 2001:a18:1:0:0:0:0:23
ns2.restena.lu. 5 AAAA 2a04:c46:e00:e7a1:477:a6ff:fe00:afb
ns3.restena.lu. 5 AAAA 2001:a18:6:2:0:0:0:45
nse.restena.lu. 5 AAAA 2a01:3f1:832:0:0:0:0:53
nsp.restena.lu. 5 AAAA 2a01:3f1:3032:0:0:0:0:53
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0