Apex domains cannot have CNAME records.
Question and response
Question
dig i-tk.de. NS
Answer
i-tk.de. 5 NS ns71.cloudns.net.
i-tk.de. 5 NS ns74.cloudns.uk.
i-tk.de. 5 NS ns73.cloudns.net.
i-tk.de. 5 NS ns72.cloudns.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0