Apex domains cannot have CNAME records.
Question and response
Question
dig csl.de. NS
Answer
csl.de. 5 NS c.ns.nrw.net.
csl.de. 5 NS b.ns.nrw.net.
csl.de. 5 NS a.ns.nrw.net.
Authority
The response has no authority records.
Additional
a.ns.nrw.net. 5 A 23.88.49.189
b.ns.nrw.net. 5 A 159.25.145.72
c.ns.nrw.net. 5 A 192.95.37.184
a.ns.nrw.net. 5 AAAA 2a01:4f8:c0c:165b:0:0:0:1
c.ns.nrw.net. 5 AAAA 2607:5300:203:7e6d:0:0:0:2222
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0