Apex domains cannot have CNAME records.
Question and response
Question
dig denic.de. NS
Answer
denic.de. 5 NS ns1.denic.de.
denic.de. 5 NS ns4.denic.net.
denic.de. 5 NS ns3.denic.de.
denic.de. 5 NS ns2.denic.de.
Authority
The response has no authority records.
Additional
ns1.denic.de. 5 A 77.67.63.106
ns2.denic.de. 5 A 81.91.164.6
ns3.denic.de. 5 A 195.243.137.27
ns1.denic.de. 5 AAAA 2001:668:1f:11:0:0:0:106
ns2.denic.de. 5 AAAA 2a02:568:0:2:0:0:0:54
ns3.denic.de. 5 AAAA 2003:8:14:0:0:0:0:106
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0