Apex domains cannot have CNAME records.
Question and response
Question
dig i-s.nl. NS
Answer
i-s.nl. 5 NS ns3.kpn.net.
i-s.nl. 5 NS ns02.is.nl.
i-s.nl. 5 NS ns03.is.nl.
i-s.nl. 5 NS ns13.kpn.net.
i-s.nl. 5 NS ns01.is.nl.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0