Apex domains cannot have CNAME records.
Question and response
Question
dig ktk.de. NS
Answer
ktk.de. 5 NS ns-b.ktk.io.
ktk.de. 5 NS ns-a.ktk.de.
ktk.de. 5 NS ns-c.ktk.de.
Authority
The response has no authority records.
Additional
ns-a.ktk.de. 5 A 212.7.160.5
ns-c.ktk.de. 5 A 212.7.160.11
ns-a.ktk.de. 5 AAAA 2a02:1628:0:c000:0:0:3:5
ns-c.ktk.de. 5 AAAA 2a02:1628:0:c000:0:0:3:11
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0