Apex domains cannot have CNAME records.
Question and response
Question
dig kellerbu.de. NS
Answer
kellerbu.de. 5 NS ns2.checkdomain.de.
kellerbu.de. 5 NS ns.checkdomain.de.
Authority
The response has no authority records.
Additional
ns.checkdomain.de. 5 A 109.237.143.11
ns2.checkdomain.de. 5 A 5.9.190.98
ns.checkdomain.de. 5 AAAA 2a02:2b80:101:5000:0:0:0:9
ns2.checkdomain.de. 5 AAAA 2a01:4f8:2b02:a88:0:0:0:106
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0