Apex domains cannot have CNAME records.
Question and response
Question
dig med.uni-rostock.de. NS
Answer
med.uni-rostock.de. 5 NS morty.med.uni-rostock.de.
med.uni-rostock.de. 5 NS rick.med.uni-rostock.de.
Authority
The response has no authority records.
Additional
rick.med.uni-rostock.de. 5 A 152.53.148.133
morty.med.uni-rostock.de. 5 A 91.99.157.115
rick.med.uni-rostock.de. 5 AAAA 2a00:11c0:47:17d4:0:0:0:1
morty.med.uni-rostock.de. 5 AAAA 2a01:4f8:c012:7b72:0:0:0:1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0