Apex domains cannot have CNAME records.
Question and response
Question
dig fondsfinanz.de. NS
Answer
fondsfinanz.de. 5 NS nsc4.schlundtech.de.
fondsfinanz.de. 5 NS nsa4.schlundtech.de.
fondsfinanz.de. 5 NS nsb4.schlundtech.de.
fondsfinanz.de. 5 NS nsd4.schlundtech.de.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0