Apex domains cannot have CNAME records.
Question and response
Question
dig kpnwerkplek.com. NS
Answer
kpnwerkplek.com. 5 NS ns13.kpn.net.
kpnwerkplek.com. 5 NS ns3.kpn.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0