Apex domains cannot have CNAME records.
Question and response
Question
dig amk-gmbh.com. NS
Answer
amk-gmbh.com. 1 NS ns2.amk-gmbh.com.
amk-gmbh.com. 1 NS ns1.amk-gmbh.com.
Authority
The response has no authority records.
Additional
ns1.amk-gmbh.com. 1 A 199.188.236.154
ns2.amk-gmbh.com. 1 A 23.137.139.26
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0