Apex domains cannot have CNAME records.
Question and response
Question
dig ipv4.wtnet.de. NS
Answer
ipv4.wtnet.de. 1 NS dns4.wtnet.de.
ipv4.wtnet.de. 1 NS dns5.wtnet.de.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0