Apex domains cannot have CNAME records.
Question and response
Question
dig uni-mainz.de. NS
Answer
Uni-Mainz.DE. 1 NS ns-extern.zdv.Uni-Mainz.DE.
Uni-Mainz.DE. 1 NS xlink1.xlink.net.
Authority
The response has no authority records.
Additional
ns-extern.zdv.uni-mainz.de. 1 A 134.93.144.7
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0