Apex domains cannot have CNAME records.
Question and response
Question
dig regzone.de. NS
Answer
regzone.de. 2 NS ns1.regzone.info.
regzone.de. 2 NS ns1.regzone.cz.
regzone.de. 2 NS ns1.regzone.de.
Authority
The response has no authority records.
Additional
ns1.regzone.cz. 2 A 217.198.113.10
ns1.regzone.de. 2 A 23.92.29.43
ns1.regzone.info. 2 A 81.0.233.10
ns1.regzone.cz. 2 AAAA 2a00:19a0:3:71:0:d9c6:710a:1
ns1.regzone.de. 2 AAAA 2600:3c02:0:0:f03c:91ff:fedb:a6e7
ns1.regzone.info. 2 AAAA 2001:1528:147:0:0:0:0:10
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0