Apex domains cannot have CNAME records.
Question and response
Question
dig mk-netzdienste.de. NS
Answer
mk-netzdienste.de. 5 NS ns1.mk.de.
mk-netzdienste.de. 5 NS ns2.mk.de.
Authority
The response has no authority records.
Additional
ns1.mk.de. 5 A 213.172.96.2
ns2.mk.de. 5 A 213.172.97.2
ns1.mk.de. 5 AAAA 2001:1640:0:0:0:0:12:30
ns2.mk.de. 5 AAAA 2001:1640:0:0:0:0:12:31
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0