Apex domains cannot have CNAME records.
Question and response
Question
dig yourvserver.net. NS
Answer
yourvserver.net. 3 NS third-dns.netcup.net.
yourvserver.net. 3 NS root-dns.netcup.net.
yourvserver.net. 3 NS second-dns.netcup.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0