Apex domains cannot have CNAME records.
Question and response
Question
dig idefender.nl. NS
Answer
idefender.nl. 1 NS ns02.ionline.nl.
idefender.nl. 1 NS ns01.ionline.nl.
Authority
The response has no authority records.
Additional
ns01.ionline.nl. 1 A 185.158.202.14
ns02.ionline.nl. 1 A 185.158.203.15
ns01.ionline.nl. 1 AAAA 2a04:c980:0:0:0:0:202:14
ns02.ionline.nl. 1 AAAA 2a04:c980:0:0:0:0:203:15
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0