Apex domains cannot have CNAME records.
Question and response
Question
dig gtnexus.com. NS
Answer
gtnexus.com. 5 NS edns2.ultradns.biz.
gtnexus.com. 5 NS edns2.ultradns.org.
gtnexus.com. 5 NS edns2.ultradns.net.
gtnexus.com. 5 NS edns2.ultradns.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0