Apex domains cannot have CNAME records.
Question and response
Question
dig test.de. NS
Answer
test.de. 4 NS dns2.iwelt-ag.net.
test.de. 4 NS dns.iwelt-ag.net.
test.de. 4 NS dns3.iwelt-ag.de.
Authority
The response has no authority records.
Additional
dns.iwelt-ag.net. 3 A 82.212.222.10
dns2.iwelt-ag.net. 3 A 82.212.255.10
dns3.iwelt-ag.de. 3 A 85.236.41.196
dns.iwelt-ag.net. 3 AAAA 2a00:cb0:1:0:0:0:0:10
dns2.iwelt-ag.net. 3 AAAA 2a00:cb0:8001:0:0:0:0:10
dns3.iwelt-ag.de. 3 AAAA 2001:4178:2:1328:0:0:0:10
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0