Apex domains cannot have CNAME records.
Question and response
Question
dig piekert.de. NS
Answer
piekert.de. 5 NS sonne.floppy.org.
piekert.de. 5 NS butterfly.post-peine.de.
piekert.de. 5 NS theater.piekert.de.
Authority
The response has no authority records.
Additional
sonne.floppy.org. 5 A 85.215.122.93
theater.piekert.de. 5 A 81.169.233.252
butterfly.post-peine.de. 5 A 81.169.217.236
theater.piekert.de. 5 AAAA 2a01:238:4381:bb00:5872:e295:9547:8ff0
butterfly.post-peine.de. 5 AAAA 2a01:238:43a1:8d00:3d66:3e7:a956:2430
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0