Apex domains cannot have CNAME records.
Question and response
Question
dig uni-paderborn.de. NS
Answer
uni-paderborn.de. 5 NS dns-1.dfn.de.
uni-paderborn.de. 5 NS dufu.uni-paderborn.de.
uni-paderborn.de. 5 NS dius.uni-paderborn.de.
Authority
The response has no authority records.
Additional
dius.uni-paderborn.de. 5 A 131.234.200.36
dufu.uni-paderborn.de. 5 A 131.234.200.52
dius.uni-paderborn.de. 5 AAAA 2001:638:502:c069:0:0:0:36
dufu.uni-paderborn.de. 5 AAAA 2001:638:502:c06a:0:0:0:52
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0