Apex domains cannot have CNAME records.
Question and response
Question
dig selbstdenker.ag. NS
Answer
selbstdenker.ag. 3 NS anycast.regdns2.net.
selbstdenker.ag. 3 NS anycast.regdns1.de.
Authority
The response has no authority records.
Additional
anycast.regdns1.de. 3 A 192.174.68.100
anycast.regdns2.net. 3 A 176.97.158.100
anycast.regdns1.de. 3 AAAA 2001:67c:1bc:0:0:0:0:100
anycast.regdns2.net. 3 AAAA 2001:67c:10b8:0:0:0:0:100
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0