Apex domains cannot have CNAME records.
Question and response
Question
dig thirdns.de. NS
Answer
thirdns.de. 5 NS acglob.thirdns.de.
thirdns.de. 5 NS acglob.firstns.cc.
thirdns.de. 5 NS acglob.secondns.at.
thirdns.de. 5 NS acglob.fourthns.systems.
thirdns.de. 5 NS acglob.fifthns.com.
Authority
The response has no authority records.
Additional
acglob.thirdns.de. 5 A 213.227.160.1
acglob.fourthns.systems. 5 A 213.227.191.1
acglob.thirdns.de. 5 AAAA 2605:380:aa1:0:0:0:0:1
acglob.fourthns.systems. 5 AAAA 2803:ad80:aa1:0:0:0:0:1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0