ns4.sidn.nl has no CNAME records.
Question and response
Question
dig ns4.sidn.nl. NS
Answer
The response has no answer records.
Authority
sidn.nl. 300 SOA ns4.sidn.nl. hostmaster.sidn.nl. 1771848536 14400 3600 3456000 300
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0