Apex domains cannot have CNAME records.
Question and response
Question
dig uni-greifswald.de. NS
Answer
uni-greifswald.de. 5 NS dns2.uni-greifswald.de.
uni-greifswald.de. 5 NS dns1.uni-greifswald.de.
Authority
The response has no authority records.
Additional
dns1.uni-greifswald.de. 5 A 141.53.10.4
dns2.uni-greifswald.de. 5 A 141.53.10.5
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0