Apex domains cannot have CNAME records.
Question and response
Question
dig convento.de. NS
Answer
convento.de. 5 NS x.ns.joker.com.
convento.de. 5 NS y.ns.joker.com.
convento.de. 5 NS z.ns.joker.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0