Apex domains cannot have CNAME records.
Question and response
Question
dig htwg-konstanz.de. NS
Answer
htwg-konstanz.de. 5 NS dns1.belwue.de.
htwg-konstanz.de. 5 NS dns5.belwue.de.
htwg-konstanz.de. 5 NS dns3.belwue.de.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0