Apex domains cannot have CNAME records.
Question and response
Question
dig teamnet.de. NS
Answer
teamnet.de. 2 NS ns5.dnsmadeeasy.com.
teamnet.de. 2 NS ns3.teamnet.de.
teamnet.de. 2 NS ns1.teamnet.de.
teamnet.de. 2 NS ns6.dnsmadeeasy.com.
teamnet.de. 2 NS ns7.dnsmadeeasy.com.
Authority
The response has no authority records.
Additional
ns1.teamnet.de. 2 A 80.87.112.130
ns3.teamnet.de. 2 A 80.87.114.2
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0