Apex domains cannot have CNAME records.
Question and response
Question
dig acumulus.nl. NS
Answer
acumulus.nl. 5 NS ns1.sielsystems.nl.
acumulus.nl. 5 NS ns2.sielsystems.nl.
Authority
The response has no authority records.
Additional
ns1.sielsystems.nl. 5 A 31.134.204.93
ns2.sielsystems.nl. 5 A 31.134.204.94
ns1.sielsystems.nl. 5 AAAA 2a12:4180:dc1:886:0:0:0:93
ns2.sielsystems.nl. 5 AAAA 2a12:4180:dc1:886:0:0:0:94
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0