Apex domains cannot have CNAME records.
Question and response
Question
dig sontheimer.de. NS
Answer
sontheimer.de. 5 NS ns2.sontheimer.eu.
sontheimer.de. 5 NS ns1.sontheimer.de.
Authority
The response has no authority records.
Additional
ns1.sontheimer.de. 5 A 194.88.183.222
ns2.sontheimer.eu. 5 A 194.88.168.65
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0