Apex domains cannot have CNAME records.
Question and response
Question
dig secure-mx.de. NS
Answer
secure-mx.de. 5 NS dns.dns4.de.
secure-mx.de. 5 NS dns.dns3.de.
secure-mx.de. 5 NS dns.dns1.de.
secure-mx.de. 5 NS dns.dns2.de.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0