Apex domains cannot have CNAME records.
Question and response
Question
dig swm.de. NS
Answer
swm.de. 5 NS ns2.m-online.net.
swm.de. 5 NS ns2.swm.de.
swm.de. 5 NS ns.swm.de.
Authority
The response has no authority records.
Additional
ns.swm.de. 5 A 188.164.233.141
ns2.swm.de. 5 A 188.164.233.142
ns2.m-online.net. 5 A 212.18.3.8
ns.swm.de. 5 AAAA 2a03:a080:200:102:0:0:0:a
ns2.swm.de. 5 AAAA 2a03:a080:200:102:0:0:0:b
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0