Apex domains cannot have CNAME records.
Question and response
Question
dig fast-ns.de. NS
Answer
fast-ns.de. 5 NS ns2.fast-ns.de.
fast-ns.de. 5 NS ns3.fast-ns.de.
fast-ns.de. 5 NS ns1.fast-ns.de.
Authority
The response has no authority records.
Additional
ns1.fast-ns.de. 5 A 37.114.61.38
ns2.fast-ns.de. 5 A 77.90.16.75
ns3.fast-ns.de. 5 A 193.32.222.80
ns1.fast-ns.de. 5 AAAA 2a12:3e80:6aa:0:1:ea7:dead:beef
ns2.fast-ns.de. 5 AAAA 2a14:7c0:7000:1a00:1:ea7:dead:bee
ns3.fast-ns.de. 5 AAAA 2a0a:51c0:0:50:0:0:0:2
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0