Apex domains cannot have CNAME records.
Question and response
Question
dig dc-frankfurt.com. NS
Answer
dc-frankfurt.com. 5 NS ns1.dc-frankfurt.com.
dc-frankfurt.com. 5 NS ns2.dc-frankfurt.com.
Authority
The response has no authority records.
Additional
ns1.dc-frankfurt.com. 5 A 185.138.143.22
ns2.dc-frankfurt.com. 5 A 185.138.142.124
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0