Apex domains cannot have CNAME records.
Question and response
Question
dig customer-net.de. NS
Answer
customer-net.de. 5 NS rdns2.internetx.de.
customer-net.de. 5 NS rdns1.internetx.de.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0