Apex domains cannot have CNAME records.
Question and response
Question
dig core.tng.de. NS
Answer
core.tng.de. 5 NS ns.ki.tng.de.
core.tng.de. 5 NS ns.tng.de.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0