Apex domains cannot have CNAME records.
Question and response
Question
dig zerfleddert.de. NS
Answer
zerfleddert.de. 5 NS agrajag.zerfleddert.de.
zerfleddert.de. 5 NS ns4.he.net.
zerfleddert.de. 5 NS ns3.he.net.
zerfleddert.de. 5 NS ns2.he.net.
zerfleddert.de. 5 NS eddie.zerfleddert.de.
Authority
The response has no authority records.
Additional
ns2.he.net. 3 A 216.218.131.2
ns3.he.net. 3 A 216.218.132.2
ns4.he.net. 3 A 216.66.1.2
eddie.zerfleddert.de. 5 A 188.246.9.226
agrajag.zerfleddert.de. 5 A 88.198.237.222
ns2.he.net. 3 AAAA 2001:470:200:0:0:0:0:2
ns3.he.net. 3 AAAA 2001:470:300:0:0:0:0:2
ns4.he.net. 3 AAAA 2001:470:400:0:0:0:0:2
eddie.zerfleddert.de. 5 AAAA 2003:a:d0a:b502:0:0:0:92
agrajag.zerfleddert.de. 5 AAAA 2a01:4f8:bc:1de:0:0:0:1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0