Apex domains cannot have CNAME records.
Question and response
Question
dig glasoperator.nl. NS
Answer
glasoperator.nl. 5 NS ns2.versatel.net.
glasoperator.nl. 5 NS ns4.versatel.net.
glasoperator.nl. 5 NS ns1.versatel.net.
glasoperator.nl. 5 NS ns3.versatel.net.
Authority
The response has no authority records.
Additional
ns1.versatel.net. 5 A 62.58.62.132
ns2.versatel.net. 5 A 62.58.94.130
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0