Apex domains cannot have CNAME records.
Question and response
Question
dig txsv.de. NS
Answer
txsv.de. 4 NS ns2.domain24.de.
txsv.de. 4 NS ns3.domain24.de.
txsv.de. 4 NS ns4.domain24.de.
txsv.de. 4 NS ns1.domain24.de.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0