Apex domains cannot have CNAME records.
Question and response
Question
dig ropadns.de. NS
Answer
ropadns.de. 5 NS ns01.ropadns.de.
ropadns.de. 5 NS ns03.ropadns.de.
ropadns.de. 5 NS ns02.ropadns.de.
Authority
The response has no authority records.
Additional
ns01.ropadns.de. 5 A 31.25.152.214
ns02.ropadns.de. 5 A 31.25.152.218
ns03.ropadns.de. 5 A 31.25.152.222
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0