Apex domains cannot have CNAME records.
Question and response
Question
dig nt-do.de. NS
Answer
nt-do.de. 5 NS ns1.goneo.de.
nt-do.de. 5 NS ns2.goneo.de.
Authority
The response has no authority records.
Additional
ns1.goneo.de. 5 A 85.220.143.1
ns2.goneo.de. 5 A 85.220.165.129
ns1.goneo.de. 5 AAAA 2001:1640:5:0:0:0:11:1
ns2.goneo.de. 5 AAAA 2001:1640:5:0:0:0:12:1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0