Apex domains cannot have CNAME records.
Question and response
Question
dig nscluster.dk. NS
Answer
nscluster.dk. 5 NS ns.nscluster.us.
nscluster.dk. 5 NS ns.nscluster.eu.
nscluster.dk. 5 NS ns.nscluster.wf.
nscluster.dk. 5 NS ns.nscluster.dk.
nscluster.dk. 5 NS ns.nscluster.uk.
Authority
The response has no authority records.
Additional
ns.nscluster.dk. 5 A 192.99.182.49
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.wf. 5 A 45.153.185.71
ns.nscluster.dk. 5 AAAA 2607:5300:205:200:0:0:0:7f7d
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