Apex domains cannot have CNAME records.
Question and response
Question
dig expurgate.de. NS
Answer
expurgate.de. 5 NS ns2.expurgate.de.
expurgate.de. 5 NS ns3.expurgate.cloud.
expurgate.de. 5 NS ns1.expurgate.net.
Authority
The response has no authority records.
Additional
ns2.expurgate.de. 5 A 202.61.229.33
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0