Apex domains cannot have CNAME records.
Question and response
Question
dig schwitzing.de. NS
Answer
schwitzing.de. 5 NS ns3.knipp.de.
schwitzing.de. 5 NS ns2.knipp.de.
schwitzing.de. 5 NS ns17.knipp.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0