Apex domains cannot have CNAME records.
Question and response
Question
dig polver.uni-konstanz.de. NS
Answer
polver.uni-konstanz.de. 5 NS ns3.uni-konstanz.de.
polver.uni-konstanz.de. 5 NS dns3.uni-konstanz.de.
polver.uni-konstanz.de. 5 NS ns4.uni-konstanz.de.
Authority
The response has no authority records.
Additional
ns3.uni-konstanz.de. 5 A 134.34.242.3
ns4.uni-konstanz.de. 5 A 134.34.242.4
dns3.uni-konstanz.de. 5 A 141.37.18.100
ns3.uni-konstanz.de. 5 AAAA 2001:7c0:2880:2010:0:0:31:3
ns4.uni-konstanz.de. 5 AAAA 2001:7c0:2880:2010:0:0:31:4
dns3.uni-konstanz.de. 5 AAAA 2001:7c0:5f0:f504:0:0:18:100
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0