Apex domains cannot have CNAME records.
Question and response
Question
dig Uni-Mainz.DE. NS
Answer
uni-mainz.de. 2 NS xlink1.xlink.net.
uni-mainz.de. 2 NS ns-extern.zdv.Uni-Mainz.DE.
Authority
The response has no authority records.
Additional
xlink1.xlink.net. 3 A 194.45.97.99
ns-extern.zdv.uni-mainz.de. 2 A 134.93.144.7
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0