Apex domains cannot have CNAME records.
Question and response
Question
dig de-nserver.de. NS
Answer
de-nserver.de. 5 NS ns1.de-nserver.de.
de-nserver.de. 5 NS ns2.de-nserver.de.
Authority
The response has no authority records.
Additional
ns1.de-nserver.de. 5 A 109.237.143.9
ns2.de-nserver.de. 5 A 5.9.190.96
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0