Apex domains cannot have CNAME records.
Question and response
Question
dig BelWue.de. NS
Answer
BelWue.de. 5 NS ns10.univie.ac.at.
BelWue.de. 5 NS dns3.BelWue.de.
BelWue.de. 5 NS pns.dtag.de.
BelWue.de. 5 NS dns1.BelWue.de.
BelWue.de. 5 NS dns-srv1.uni-stuttgart.de.
BelWue.de. 5 NS ns5.univie.ac.at.
BelWue.de. 5 NS dns5.BelWue.de.
Authority
The response has no authority records.
Additional
pns.dtag.de. 5 A 194.25.0.125
dns1.belwue.de. 5 A 129.143.2.10
dns3.belwue.de. 5 A 129.143.253.133
dns5.belwue.de. 5 A 129.143.4.5
pns.dtag.de. 5 AAAA 2003:40:8000:0:0:0:0:100
dns1.belwue.de. 5 AAAA 2001:7c0:0:0:0:0:53:10
dns3.belwue.de. 5 AAAA 2001:7c0:0:253:0:0:0:133
dns5.belwue.de. 5 AAAA 2001:7c0:0:0:0:0:53:5
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0