Apex domains cannot have CNAME records.
Question and response
Question
dig stiftung-warentest.de. NS
Answer
stiftung-warentest.de. 5 NS anycast.iwelt.de.
stiftung-warentest.de. 5 NS anycast2.iwelt.domains.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0