Apex domains cannot have CNAME records.
Question and response
Question
dig belwue.DE. NS
Answer
BelWue.de. 2 NS dns3.belwue.de.
BelWue.de. 2 NS dns5.belwue.de.
BelWue.de. 2 NS dns1.belwue.de.
BelWue.de. 2 NS pns.dtag.de.
BelWue.de. 2 NS ns5.univie.ac.at.
BelWue.de. 2 NS dns-srv1.uni-stuttgart.de.
BelWue.de. 2 NS ns10.univie.ac.at.
Authority
The response has no authority records.
Additional
ns5.univie.ac.at. 5 A 193.171.255.77
pns.dtag.de. 2 A 194.25.0.125
dns1.belwue.de. 3 A 129.143.2.10
dns3.belwue.de. 3 A 129.143.253.133
dns5.belwue.de. 2 A 129.143.4.5
ns10.univie.ac.at. 5 A 192.76.243.2
dns-srv1.uni-stuttgart.de. 5 A 129.69.252.130
ns5.univie.ac.at. 5 AAAA 2001:628:453:4305:0:0:0:53
pns.dtag.de. 2 AAAA 2003:40:8000:0:0:0:0:100
dns1.belwue.de. 3 AAAA 2001:7c0:0:0:0:0:53:10
dns3.belwue.de. 3 AAAA 2001:7c0:0:253:0:0:0:133
dns5.belwue.de. 2 AAAA 2001:7c0:0:0:0:0:53:5
ns10.univie.ac.at. 5 AAAA 2001:67c:133c:0:0:0:0:2
dns-srv1.uni-stuttgart.de. 5 AAAA 2001:7c0:2053:5301:0:0:0:2
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0