Apex domains cannot have CNAME records.
Question and response
Question
dig vtmgroep.nl. NS
Answer
vtmgroep.nl. 5 NS ns2.snel.com.
vtmgroep.nl. 5 NS ns1.snel.com.
vtmgroep.nl. 5 NS ns3.snel.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0