Apex domains cannot have CNAME records.
Question and response
Question
dig greencells.net. NS
Answer
greencells.net. 5 NS ns1038.ui-dns.de.
greencells.net. 5 NS ns1051.ui-dns.biz.
greencells.net. 5 NS ns1079.ui-dns.com.
greencells.net. 5 NS ns1108.ui-dns.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0