Apex domains cannot have CNAME records.
Question and response
Question
dig domain24.de. NS
Answer
domain24.de. 4 NS ns2.domain24.com.
domain24.de. 4 NS ns1.domain24.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0