Apex domains cannot have CNAME records.
Question and response
Question
dig rocketno.de. NS
Answer
rocketno.de. 5 NS ian.ns.cloudflare.com.
rocketno.de. 5 NS lady.ns.cloudflare.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0