Apex domains cannot have CNAME records.
Question and response
Question
dig tik.uni-stuttgart.de. NS
Answer
tik.uni-stuttgart.de. 5 NS dns-auth3.uni-stuttgart.net.
tik.uni-stuttgart.de. 5 NS dns-auth1.uni-stuttgart.de.
tik.uni-stuttgart.de. 5 NS dns-auth2.uni-stuttgart.edu.
Authority
The response has no authority records.
Additional
dns-auth1.uni-stuttgart.de. 5 A 129.69.252.129
dns-auth3.uni-stuttgart.net. 5 A 129.69.252.153
dns-auth1.uni-stuttgart.de. 5 AAAA 2001:7c0:2053:5301:0:0:0:1
dns-auth3.uni-stuttgart.net. 5 AAAA 2001:7c0:2053:5303:0:0:0:1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0