Apex domains cannot have CNAME records.
Question and response
Question
dig m-net.de. NS
Answer
m-net.de. 5 NS ns3.m-online.net.
m-net.de. 5 NS ns2.m-online.net.
m-net.de. 5 NS ns1.m-online.net.
m-net.de. 5 NS ns4.m-online.net.
Authority
The response has no authority records.
Additional
ns1.m-online.net. 5 A 212.18.0.8
ns2.m-online.net. 5 A 212.18.3.8
ns3.m-online.net. 5 A 217.160.41.67
ns4.m-online.net. 5 A 212.114.171.64
ns1.m-online.net. 5 AAAA 2001:a60:0:11:0:0:0:53
ns3.m-online.net. 5 AAAA 2001:8d8:1801:8039:0:0:0:1
ns4.m-online.net. 5 AAAA 2001:a60:0:20:0:0:0:53
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0