Apex domains cannot have CNAME records.
Question and response
Question
dig stb-in-ol.de. NS
Answer
stb-in-ol.de. 5 NS ns3.ai-dns.de.
stb-in-ol.de. 5 NS ns6.kasserver.com.
stb-in-ol.de. 5 NS ns5.name-server-kas.de.
stb-in-ol.de. 5 NS ns5.kasserver.com.
Authority
The response has no authority records.
Additional
ns5.kasserver.com. 5 A 85.13.128.3
ns6.kasserver.com. 5 A 85.13.159.101
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0