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 dns-2.dfn.de.
berlin.ptb.de. 5 NS ns1.ptb.de.
berlin.ptb.de. 5 NS dns-1.dfn.de.
Authority
The response has no authority records.
Additional
ns1.ptb.de. 5 A 192.53.103.180
ns1.ptb.de. 5 AAAA 2001:638:610:bd02:0:0:0:14
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0