Apex domains cannot have CNAME records.
Question and response
Question
dig sg.de.o2.com. NS
Answer
sg.de.o2.com. 5 NS ns1.o2online.de.
sg.de.o2.com. 5 NS ns2.o2online.de.
sg.de.o2.com. 5 NS ns3.o2online.de.
Authority
The response has no authority records.
Additional
ns1.o2online.de. 5 A 82.113.113.132
ns2.o2online.de. 5 A 82.113.113.150
ns3.o2online.de. 5 A 213.20.248.180
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0