Apex domains cannot have CNAME records.
Question and response
Question
dig signet.nl. NS
Answer
signet.nl. 5 NS nsauth2.signet.nl.
signet.nl. 5 NS nsauth1.signet.nl.
signet.nl. 5 NS nsauth3.signet.nl.
Authority
The response has no authority records.
Additional
nsauth1.signet.nl. 5 A 217.21.244.30
nsauth2.signet.nl. 5 A 78.108.128.30
nsauth3.signet.nl. 5 A 217.21.244.71
nsauth1.signet.nl. 5 AAAA 2001:4cb8:0:7:0:0:53:1
nsauth2.signet.nl. 5 AAAA 2001:4cb8:1:3:0:0:53:1
nsauth3.signet.nl. 5 AAAA 2001:4cb8:0:70:0:0:53:1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0