Apex domains cannot have CNAME records.
Question and response
Question
dig uni-koeln.de. NS
Answer
uni-koeln.de. 5 NS noc2.rrz.uni-koeln.de.
uni-koeln.de. 5 NS mail1.rrz.uni-koeln.de.
uni-koeln.de. 5 NS ns2.netcologne.de.
uni-koeln.de. 5 NS dns-1.dfn.de.
Authority
The response has no authority records.
Additional
noc2.rrz.uni-koeln.de. 5 A 134.95.129.23
mail1.rrz.uni-koeln.de. 5 A 134.95.100.208
noc2.rrz.uni-koeln.de. 5 AAAA 2a00:a200:0:50:0:0:0:53
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0