Apex domains cannot have CNAME records.
Question and response
Question
dig root24.de. NS
Answer
root24.de. 5 NS ns2.root24.de.
root24.de. 5 NS ns1.root24.de.
root24.de. 5 NS ns3.root24.de.
Authority
The response has no authority records.
Additional
ns1.root24.de. 5 A 145.239.234.144
ns2.root24.de. 5 A 213.251.188.141
ns3.root24.de. 5 A 145.239.234.145
ns1.root24.de. 5 AAAA 2001:41d0:700:3394:0:0:0:1
ns2.root24.de. 5 AAAA 2001:41d0:1:4a8d:0:0:0:1
ns3.root24.de. 5 AAAA 2001:41d0:700:3394:1:0:0:2
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0