Apex domains cannot have CNAME records.
Question and response
Question
dig rackspeed.de. NS
Answer
rackspeed.de. 5 NS ns.rackspeed-dns.eu.
rackspeed.de. 5 NS ns.rackspeed-dns.net.
rackspeed.de. 5 NS ns.rackspeed-dns.de.
rackspeed.de. 5 NS ns.rackspeed-dns.com.
Authority
The response has no authority records.
Additional
ns.rackspeed-dns.de. 5 A 45.83.248.110
ns.rackspeed-dns.eu. 5 A 45.83.249.110
ns.rackspeed-dns.com. 5 A 45.83.250.110
ns.rackspeed-dns.de. 5 AAAA 2a06:fb00:2:0:0:0:1:110
ns.rackspeed-dns.eu. 5 AAAA 2a06:fb00:2:0:0:0:2:110
ns.rackspeed-dns.com. 5 AAAA 2a06:fb00:2:0:0:0:3:110
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0