Apex domains cannot have CNAME records.
Question and response
Question
dig spnr.de. NS
Answer
spnr.de. 5 NS nox.spnr.de.
spnr.de. 5 NS dns-backup.spanier.es.
spnr.de. 5 NS lum.spnr.de.
Authority
The response has no authority records.
Additional
lum.spnr.de. 5 A 176.103.220.139
nox.spnr.de. 5 A 159.195.55.239
dns-backup.spanier.es. 5 A 82.165.96.230
dns-backup.spanier.es. 5 A 212.227.189.68
lum.spnr.de. 5 AAAA 2a10:fc81:ca1c:2e2f:0:0:0:1
nox.spnr.de. 5 AAAA 2a0a:4cc0:c1:db81:0:0:0:1
dns-backup.spanier.es. 5 AAAA 2a01:239:2b5:5a00:0:0:0:1
dns-backup.spanier.es. 5 AAAA 2a01:239:3ae:f400:0:0:0:1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0