Apex domains cannot have CNAME records.
Question and response
Question
dig sbs.de. NS
Answer
sbs.de. 4 NS david.siemens.com.
sbs.de. 4 NS david.siemens.de.
sbs.de. 4 NS ns.sbs.de.
sbs.de. 4 NS david.siemens.com.sg.
Authority
The response has no authority records.
Additional
ns.sbs.de. 4 A 194.138.37.44
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0