Apex domains cannot have CNAME records.
Question and response
Question
dig gunnarhofmann.de. NS
Answer
gunnarhofmann.de. 4 NS ns5.he.net.
gunnarhofmann.de. 4 NS ns3.he.net.
gunnarhofmann.de. 4 NS ns.gunnarhofmann.de.
gunnarhofmann.de. 4 NS ns4.he.net.
Authority
The response has no authority records.
Additional
ns.gunnarhofmann.de. 4 A 176.9.157.155
ns.gunnarhofmann.de. 4 AAAA 2a01:4f8:160:43aa:0:0:0:2
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0