Apex domains cannot have CNAME records.
Question and response
Question
dig in-berlin.de. NS
Answer
in-berlin.de. 5 NS ns4.in-berlin.de.
in-berlin.de. 5 NS ns1.in-berlin.de.
in-berlin.de. 5 NS ns2.in-berlin.de.
in-berlin.de. 5 NS ns3.in-berlin.de.
Authority
The response has no authority records.
Additional
ns1.in-berlin.de. 5 A 192.109.42.26
ns2.in-berlin.de. 5 A 217.197.80.34
ns3.in-berlin.de. 5 A 193.29.188.4
ns4.in-berlin.de. 5 A 130.133.8.36
ns1.in-berlin.de. 5 AAAA 2001:67c:1400:1010:0:0:42:26
ns2.in-berlin.de. 5 AAAA 2001:67c:1400:1010:0:0:0:34
ns3.in-berlin.de. 5 AAAA 2001:67c:1403:1010:0:0:0:4
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0