Apex domains cannot have CNAME records.
Question and response
Question
dig ze.tum.de. NS
Answer
ze.tum.de. 5 NS w3projns.ze.tum.de.
ze.tum.de. 5 NS dns3.lrz.eu.
ze.tum.de. 5 NS dns1.lrz.de.
ze.tum.de. 5 NS dns2.lrz.bayern.
Authority
The response has no authority records.
Additional
dns1.lrz.de. 1 A 129.187.19.183
dns2.lrz.bayern. 1 A 141.40.9.211
dns3.lrz.eu. 1 A 78.128.211.180
dns1.lrz.de. 1 AAAA 2001:4ca0:0:100:0:53:1:1
dns2.lrz.bayern. 1 AAAA 2001:4ca0:0:100:0:53:1:2
dns3.lrz.eu. 1 AAAA 2001:718:1:1f:50:56ff:feee:180
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0