Apex domains cannot have CNAME records.
Question and response
Question
dig uni-rostock.de. NS
Answer
uni-rostock.de. 5 NS dns-3.dfn.de.
uni-rostock.de. 5 NS dns-2.dfn.de.
uni-rostock.de. 5 NS george.ns.uni-rostock.de.
uni-rostock.de. 5 NS ringo.ns.uni-rostock.de.
Authority
The response has no authority records.
Additional
ringo.ns.uni-rostock.de. 5 A 139.30.255.117
george.ns.uni-rostock.de. 5 A 139.30.255.118
ringo.ns.uni-rostock.de. 5 AAAA 2001:638:804:bf:0:0:0:117
george.ns.uni-rostock.de. 5 AAAA 2001:638:804:bf:0:0:0:118
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0