Apex domains cannot have CNAME records.
Question and response
Question
dig keyweb.de. NS
Answer
keyweb.de. 5 NS nitin.ns.cloudflare.com.
keyweb.de. 5 NS zita.ns.cloudflare.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0