Apex domains cannot have CNAME records.
Question and response
Question
dig buysub.com. NS
Answer
buysub.com. 5 NS pdns67.ultradns.biz.
buysub.com. 5 NS pdns67.ultradns.com.
buysub.com. 5 NS pdns67.ultradns.net.
buysub.com. 5 NS authns1.centurylink.net.
buysub.com. 5 NS pdns67.ultradns.info.
buysub.com. 5 NS pdns67.ultradns.co.uk.
buysub.com. 5 NS authns2.centurylink.net.
buysub.com. 5 NS pdns67.ultradns.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0