Apex domains cannot have CNAME records.
Question and response
Question
dig slu-gmbh.de. NS
Answer
slu-gmbh.de. 5 NS b.regfish-ns.net.
slu-gmbh.de. 5 NS a.regfish-ns.net.
slu-gmbh.de. 5 NS c.regfish-ns.net.
Authority
The response has no authority records.
Additional
a.regfish-ns.net. 5 A 178.21.144.21
b.regfish-ns.net. 5 A 178.21.145.21
c.regfish-ns.net. 5 A 178.21.146.21
a.regfish-ns.net. 5 AAAA 2a02:2c0:0:21:0:0:aced:beef
b.regfish-ns.net. 5 AAAA 2a02:2c0:1:21:0:0:aced:beef
c.regfish-ns.net. 5 AAAA 2a02:2c0:2:21:0:0:aced:beef
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0