Apex domains cannot have CNAME records.
Question and response
Question
dig boxspringbett-kaufen.com. NS
Answer
boxspringbett-kaufen.com. 5 NS ns-1134.awsdns-13.org.
boxspringbett-kaufen.com. 5 NS ns-996.awsdns-60.net.
boxspringbett-kaufen.com. 5 NS ns-216.awsdns-27.com.
boxspringbett-kaufen.com. 5 NS ns-1684.awsdns-18.co.uk.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0