Apex domains cannot have CNAME records.
Question and response
Question
dig glb.proximus.be. NS
Answer
glb.proximus.be. 5 NS dfonemesgtm2.glb.proximus.be.
glb.proximus.be. 5 NS dfonegamgtm2.glb.proximus.be.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0