Apex domains cannot have CNAME records.
Question and response
Question
dig bithal.de. NS
Answer
bithal.de. 5 NS ns.kellerbu.de.
bithal.de. 5 NS ns2.kellerbu.de.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0