Apex domains cannot have CNAME records.
Question and response
Question
dig uni-ulm.de. NS
Answer
uni-ulm.de. 5 NS dns1.belwue.de.
uni-ulm.de. 5 NS dns5.belwue.de.
uni-ulm.de. 5 NS dns3.belwue.de.
uni-ulm.de. 5 NS dns1.uni-ulm.de.
uni-ulm.de. 5 NS dns-1.dfn.de.
Authority
The response has no authority records.
Additional
dns1.belwue.de. 5 A 129.143.2.10
dns1.uni-ulm.de. 5 A 134.60.1.112
dns3.belwue.de. 5 A 129.143.253.133
dns5.belwue.de. 5 A 129.143.4.5
dns1.belwue.de. 5 AAAA 2001:7c0:0:0:0:0:53:10
dns1.uni-ulm.de. 5 AAAA 2001:7c0:3100:0:0:0:0:112
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