Apex domains cannot have CNAME records.
Question and response
Question
dig quattroxx.de. NS
Answer
quattroxx.de. 1 NS helium.ns.hetzner.de.
quattroxx.de. 1 NS oxygen.ns.hetzner.com.
quattroxx.de. 1 NS hydrogen.ns.hetzner.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0