Apex domains cannot have CNAME records.
Question and response
Question
dig lewron.de. NS
Answer
lewron.de. 5 NS shades20.rzone.de.
lewron.de. 5 NS docks03.rzone.de.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0