Apex domains cannot have CNAME records.
Question and response
Question
dig uoc.gr. NS
Answer
uoc.gr. 5 NS sns0.grnet.gr.
uoc.gr. 5 NS ns2.uoc.gr.
uoc.gr. 5 NS ns.uoc.gr.
uoc.gr. 5 NS sns1.grnet.gr.
uoc.gr. 5 NS estia.csi.forth.gr.
Authority
The response has no authority records.
Additional
ns.uoc.gr. 5 A 147.52.80.1
ns2.uoc.gr. 5 A 147.52.81.140
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0