Apex domains cannot have CNAME records.
Question and response
Question
dig proservers.nl. NS
Answer
proservers.nl. 5 NS ns.pro-serve.eu.
proservers.nl. 5 NS ns.proserve.nl.
proservers.nl. 5 NS ns.proserve.org.
proservers.nl. 5 NS ns.pro-serve.be.
Authority
The response has no authority records.
Additional
ns.proserve.nl. 5 A 18.185.250.139
ns.proserve.org. 5 A 81.4.77.196
ns.pro-serve.be. 5 A 3.255.55.103
ns.pro-serve.eu. 5 A 80.84.227.131
ns.proserve.nl. 5 AAAA 2a05:d014:1c00:3a01:fd5:10dd:4412:e478
ns.proserve.org. 5 AAAA 2001:828:13e9:1000:196:0:0:1
ns.pro-serve.be. 5 AAAA 2a05:d018:746:6e01:d78a:2cec:2a57:9d57
ns.pro-serve.eu. 5 AAAA 2001:828:13e9:1:131:0:0:1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0