Apex domains cannot have CNAME records.
Question and response
Question
dig bert.group. NS
Answer
bert.group. 5 NS ns1.arvato-systems.de.
bert.group. 5 NS ns2.arvato-systems.de.
bert.group. 5 NS dns009.arvato-systems.de.
bert.group. 5 NS dns017.arvato-systems.de.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0