Apex domains cannot have CNAME records.
Question and response
Question
dig mx-rec.de. NS
Answer
mx-rec.de. 5 NS oxygen.ns.hetzner.com.
mx-rec.de. 5 NS hydrogen.ns.hetzner.com.
mx-rec.de. 5 NS helium.ns.hetzner.de.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0