Apex domains cannot have CNAME records.
Question and response
Question
dig selfhost.de. NS
Answer
selfhost.de. 5 NS sec.selfhost.de.
selfhost.de. 5 NS pri.selfhost.de.
Authority
The response has no authority records.
Additional
pri.selfhost.de. 5 A 82.98.82.20
sec.selfhost.de. 5 A 185.26.156.9
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0