Apex domains cannot have CNAME records.
Question and response
Question
dig uni-bielefeld.de. NS
Answer
uni-bielefeld.de. 5 NS ns1.uni-bielefeld.de.
uni-bielefeld.de. 5 NS ns2.uni-bielefeld.de.
uni-bielefeld.de. 5 NS dns-2.dfn.de.
Authority
The response has no authority records.
Additional
ns1.uni-bielefeld.de. 5 A 129.70.182.7
ns2.uni-bielefeld.de. 5 A 129.70.182.23
ns1.uni-bielefeld.de. 5 AAAA 2001:638:504:8204:0:0:0:7
ns2.uni-bielefeld.de. 5 AAAA 2001:638:504:8204:0:0:0:23
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0