Apex domains cannot have CNAME records.
Question and response
Question
dig tu-bs.de. NS
Answer
tu-bs.de. 5 NS rzdnsprime.rz.tu-bs.de.
tu-bs.de. 5 NS rzdnssecond.rz.tu-bs.de.
tu-bs.de. 5 NS deneb.dfn.de.
Authority
The response has no authority records.
Additional
rzdnsprime.rz.tu-bs.de. 5 A 134.169.12.124
rzdnssecond.rz.tu-bs.de. 5 A 134.169.2.234
rzdnssecond.rz.tu-bs.de. 5 AAAA 2001:638:602:1100:0:0:0:3
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0