Apex domains cannot have CNAME records.
Question and response
Question
dig dns-net.de. NS
Answer
dns-net.de. 4 NS ns7.dns-net.de.
dns-net.de. 4 NS ns0.dns-net.de.
Authority
The response has no authority records.
Additional
ns0.dns-net.de. 4 A 212.91.230.34
ns7.dns-net.de. 4 A 212.91.231.34
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0