Apex domains cannot have CNAME records.
Question and response
Question
dig berlin.PTB.DE. NS
Answer
berlin.ptb.de. 5 NS dns-3.dfn.de.
berlin.ptb.de. 5 NS ns1.ptb.de.
berlin.ptb.de. 5 NS dns-1.dfn.de.
berlin.ptb.de. 5 NS dns-2.dfn.de.
Authority
The response has no authority records.
Additional
ns1.PTB.DE. 5 A 192.53.103.180
dns-1.dfn.de. 3 A 193.174.75.50
dns-2.dfn.de. 3 A 193.174.75.54
dns-3.dfn.de. 3 A 193.174.75.58
ns1.PTB.DE. 5 AAAA 2001:638:610:bd02:0:0:0:14
dns-1.dfn.de. 3 AAAA 2001:638:d:b101:0:0:0:1
dns-2.dfn.de. 3 AAAA 2001:638:d:b102:0:0:0:1
dns-3.dfn.de. 3 AAAA 2001:638:d:b103:0:0:0:1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0