Apex domains cannot have CNAME records.
Question and response
Question
dig bgl.lu. NS
Answer
bgl.lu. 5 NS ns3.domivesta.net.
bgl.lu. 5 NS ns2.bnpparibas.com.
bgl.lu. 5 NS ns1.bnpparibas.com.
bgl.lu. 5 NS ns4.domivesta.com.
Authority
The response has no authority records.
Additional
ns1.bnpparibas.com. 5 A 193.108.91.9
ns2.bnpparibas.com. 5 A 2.22.230.67
ns3.domivesta.net. 5 A 96.7.50.66
ns4.domivesta.com. 5 A 23.74.25.65
ns1.bnpparibas.com. 5 AAAA 2600:1401:2:0:0:0:0:9
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0