Apex domains cannot have CNAME records.
Question and response
Question
dig eduroam.lu. NS
Answer
eduroam.lu. 5 NS ns2.restena.lu.
eduroam.lu. 5 NS ns1.restena.lu.
eduroam.lu. 5 NS ns3.restena.lu.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0