Apex domains cannot have CNAME records.
Question and response
Question
dig documbox.com. NS
Answer
documbox.com. 3 NS ns-762.awsdns-31.net.
documbox.com. 3 NS ns-257.awsdns-32.com.
documbox.com. 3 NS ns-513.awsdns-00.net.
documbox.com. 3 NS ns-2035.awsdns-62.co.uk.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0