Apex domains cannot have CNAME records.
Question and response
Question
dig askja.de. NS
Answer
askja.de. 2 NS ns.askja.de.
askja.de. 2 NS ns1.askja.de.
Authority
The response has no authority records.
Additional
ns.askja.de. 2 A 80.82.204.170
ns1.askja.de. 2 A 85.17.15.98
ns.askja.de. 2 AAAA 2a00:cd0:1016:1:0:0:170:1
ns1.askja.de. 2 AAAA 2001:1af8:4700:a119:6:0:0:1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0