Apex domains cannot have CNAME records.
Question and response
Question
dig solidotel.lu. NS
Answer
solidotel.lu. 5 NS nspub-eu.dns.lu.
solidotel.lu. 5 NS nspub2.dns.lu.
solidotel.lu. 5 NS nspub1.dns.lu.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0