Apex domains cannot have CNAME records.
Question and response
Question
dig sidn.nl. NS
Answer
sidn.nl. 5 NS ns5.sidn.nl.
sidn.nl. 5 NS ns3.sidn.nl.
sidn.nl. 5 NS ns4.sidn.nl.
Authority
The response has no authority records.
Additional
ns3.sidn.nl. 5 A 194.0.28.10
ns4.sidn.nl. 5 A 212.114.120.108
ns5.sidn.nl. 5 A 147.75.205.55
ns3.sidn.nl. 5 AAAA 2001:678:2c:0:194:0:28:10
ns4.sidn.nl. 5 AAAA 2001:7b8:62b:1:0:d4ff:fe72:786c
ns5.sidn.nl. 5 AAAA 2604:1380:4601:5c00:0:0:0:d
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0