Apex domains cannot have CNAME records.
Question and response
Question
dig fortis.lu. NS
Answer
fortis.lu. 5 NS ns-135-c.gandi.net.
fortis.lu. 5 NS ns-180-b.gandi.net.
fortis.lu. 5 NS ns-195-a.gandi.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0