Apex domains cannot have CNAME records.
Question and response
Question
dig a4a.de. NS
Answer
a4a.de. 5 NS ns1.desec.io.
a4a.de. 5 NS ns2.desec.org.
Authority
The response has no authority records.
Additional
ns1.desec.io. 5 A 45.54.76.1
ns2.desec.org. 5 A 157.53.224.1
ns1.desec.io. 5 AAAA 2607:f740:e633:deec:0:0:0:2
ns2.desec.org. 5 AAAA 2607:f740:e00a:deec:0:0:0:2
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0