Apex domains cannot have CNAME records.
Question and response
Question
dig nscluster.de. NS
Answer
nscluster.de. 5 NS ns.nscluster.de.
nscluster.de. 5 NS ns.nscluster.us.
nscluster.de. 5 NS ns.nscluster.uk.
nscluster.de. 5 NS ns.nscluster.be.
nscluster.de. 5 NS ns.nscluster.eu.
Authority
The response has no authority records.
Additional
ns.nscluster.be. 5 A 144.217.189.222
ns.nscluster.de. 5 A 137.74.111.240
ns.nscluster.eu. 5 A 188.166.70.123
ns.nscluster.uk. 5 A 13.248.156.209
ns.nscluster.us. 5 A 198.251.86.156
ns.nscluster.be. 5 AAAA 2607:5300:201:3100:0:0:0:92af
ns.nscluster.de. 5 AAAA 2001:41d0:404:200:0:0:0:2fbb
ns.nscluster.eu. 5 AAAA 2a03:b0c0:2:d0:0:0:79d:4001
ns.nscluster.uk. 5 AAAA 2600:9000:a51d:1c55:a63e:443d:ec03:df01
ns.nscluster.us. 5 AAAA 2605:6400:10:5c1:84ee:85fb:b2b:8b37
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0