Apex domains cannot have CNAME records.
Question and response
Question
dig htp-tel.de. NS
Answer
htp-tel.de. 5 NS ns2.htp-tel.de.
htp-tel.de. 5 NS ns1.htp-tel.de.
htp-tel.de. 5 NS ns.htp-tel.de.
Authority
The response has no authority records.
Additional
ns.htp-tel.de. 5 A 81.14.244.10
ns1.htp-tel.de. 5 A 81.14.243.10
ns2.htp-tel.de. 5 A 212.59.54.188
ns.htp-tel.de. 5 AAAA 2a02:560:2:140:0:0:0:10
ns1.htp-tel.de. 5 AAAA 2a02:560:2:100:0:0:0:10
ns2.htp-tel.de. 5 AAAA 2a02:560:2:180:0:0:0:10
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0