Apex domains cannot have CNAME records.
Question and response
Question
dig isite.de. NS
Answer
isite.de. 5 NS dns3.iwelt-ag.de.
isite.de. 5 NS dns.iwelt-ag.net.
isite.de. 5 NS dns2.iwelt-ag.net.
Authority
The response has no authority records.
Additional
dns.iwelt-ag.net. 5 A 82.212.222.10
dns2.iwelt-ag.net. 5 A 82.212.255.10
dns3.iwelt-ag.de. 5 A 85.236.41.196
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0