Apex domains cannot have CNAME records.
Question and response
Question
dig c-388.maxcluster.net. NS
Answer
c-388.maxcluster.net. 5 NS nameserver-1-de.maxcluster.net.
c-388.maxcluster.net. 5 NS nameserver-2-de.maxcluster.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0