Apex domains cannot have CNAME records.
Question and response
Question
dig knipp.de. NS
Answer
knipp.de. 5 NS ns8.knipp.net.
knipp.de. 5 NS ns90.knipp.de.
knipp.de. 5 NS ns0.knipp.de.
knipp.de. 5 NS ns7.knipp.de.
Authority
The response has no authority records.
Additional
ns0.knipp.de. 5 A 195.253.51.63
ns7.knipp.de. 5 A 195.253.6.76
ns90.knipp.de. 5 A 195.253.102.90
ns0.knipp.de. 5 AAAA 2a01:5b0:2:56:0:0:0:3f
ns7.knipp.de. 5 AAAA 2a01:5b0:0:29:0:0:0:4c
ns90.knipp.de. 5 AAAA 2a01:5b0:26:0:0:0:0:5a
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0