Apex domains cannot have CNAME records.
Question and response
Question
dig secondary-nameserver.de. NS
Answer
secondary-nameserver.de. 5 NS secondary-nameserver.de.
secondary-nameserver.de. 5 NS primary-nameserver.de.
Authority
The response has no authority records.
Additional
secondary-nameserver.de. 5 A 5.161.181.129
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0