Apex domains cannot have CNAME records.
Question and response
Question
dig madduck.net. NS
Answer
madduck.net. 5 NS robotns2.second-ns.de.
madduck.net. 5 NS robotns3.second-ns.com.
madduck.net. 5 NS a.ns.madduck.net.
madduck.net. 5 NS ns1.first-ns.de.
Authority
The response has no authority records.
Additional
a.ns.madduck.net. 5 A 188.174.253.164
ns1.first-ns.de. 1 A 213.239.242.238
robotns2.second-ns.de. 1 A 213.133.100.103
a.ns.madduck.net. 5 AAAA 2001:a60:902f:0:0:53:0:0
ns1.first-ns.de. 1 AAAA 2a01:4f8:0:a101:0:0:a:1
robotns2.second-ns.de. 1 AAAA 2a01:4f8:0:1:0:0:5ddc:2
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0