Apex domains cannot have CNAME records.
Question and response
Question
dig speedbone.de. NS
Answer
speedbone.de. 3 NS ns2.speedbone.de.
speedbone.de. 3 NS ns1.speedbone.de.
Authority
The response has no authority records.
Additional
ns1.speedbone.de. 3 A 217.13.192.3
ns2.speedbone.de. 3 A 217.13.193.3
ns1.speedbone.de. 3 AAAA 2a02:8b8:704:0:0:0:0:3001
ns2.speedbone.de. 3 AAAA 2a02:8b8:704:0:0:0:0:3002
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0