Apex domains cannot have CNAME records.
Question and response
Question
dig nsentry.de. NS
Answer
nsentry.de. 5 NS ns2.nsentry.de.
nsentry.de. 5 NS ns1.nsentry.de.
Authority
The response has no authority records.
Additional
ns1.nsentry.de. 5 A 212.162.15.5
ns2.nsentry.de. 5 A 213.203.243.12
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0