Apex domains cannot have CNAME records.
Question and response
Question
dig converia.de. NS
Answer
converia.de. 5 NS nsc9.schlundtech.de.
converia.de. 5 NS nsd9.schlundtech.de.
converia.de. 5 NS nsa9.schlundtech.de.
converia.de. 5 NS nsb9.schlundtech.de.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0