Apex domains cannot have CNAME records.
Question and response
Question
dig nexnet.de. NS
Answer
nexnet.de. 5 NS ns.nexnet.de.
nexnet.de. 5 NS pdns02.domaincontrol.com.
Authority
The response has no authority records.
Additional
ns.nexnet.de. 5 A 92.205.233.126
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0