Apex domains cannot have CNAME records.
Question and response
Question
dig plus.net. NS
Answer
plus.net. 5 NS ns2.force9.net.
plus.net. 5 NS ns1.force9.net.
Authority
The response has no authority records.
Additional
ns1.force9.net. 5 A 195.166.128.16
ns2.force9.net. 5 A 195.166.128.17
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0