Apex domains cannot have CNAME records.
Question and response
Question
dig fastwebserver.de. NS
Answer
fastwebserver.de. 5 NS ns3.ntdns.de.
fastwebserver.de. 5 NS ns1.ntdns.de.
fastwebserver.de. 5 NS ns2.ntdns.de.
Authority
The response has no authority records.
Additional
ns1.ntdns.de. 5 A 62.141.32.2
ns2.ntdns.de. 5 A 62.141.34.2
ns3.ntdns.de. 5 A 217.79.176.138
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0