Apex domains cannot have CNAME records.
Question and response
Question
dig secova.de. NS
Answer
secova.de. 5 NS ns-1945.awsdns-51.co.uk.
secova.de. 5 NS ns-566.awsdns-06.net.
secova.de. 5 NS ns-91.awsdns-11.com.
secova.de. 5 NS ns-1333.awsdns-38.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0