Apex domains cannot have CNAME records.
Question and response
Question
dig uni-erlangen.de. NS
Answer
uni-erlangen.de. 5 NS dns-3.dfn.de.
uni-erlangen.de. 5 NS ns2.rrze.uni-erlangen.de.
uni-erlangen.de. 5 NS ns1.rrze.uni-erlangen.de.
uni-erlangen.de. 5 NS tuminfo1.informatik.tu-muenchen.de.
Authority
The response has no authority records.
Additional
ns1.rrze.uni-erlangen.de. 5 A 131.188.3.2
ns2.rrze.uni-erlangen.de. 5 A 131.188.12.100
ns1.rrze.uni-erlangen.de. 5 AAAA 2001:638:a000:1001:53:0:0:1
ns2.rrze.uni-erlangen.de. 5 AAAA 2001:638:a000:1054:53:0:0:2
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0