Apex domains cannot have CNAME records.
Question and response
Question
dig sda.t-online.de. NS
Answer
sda.t-online.de. 5 NS dns01.sul.t-online.de.
sda.t-online.de. 5 NS dns00.sul.t-online.de.
sda.t-online.de. 5 NS dns00.sda.t-online.de.
sda.t-online.de. 5 NS dns01.sda.t-online.de.
Authority
The response has no authority records.
Additional
dns00.sda.t-online.de. 5 A 195.145.119.62
dns01.sda.t-online.de. 5 A 195.145.119.189
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0