Apex domains cannot have CNAME records.
Question and response
Question
dig bmc.com. NS
Answer
bmc.com. 5 NS pdns6.ultradns.co.uk.
bmc.com. 5 NS pdns5.ultradns.info.
bmc.com. 5 NS pdns1.ultradns.net.
bmc.com. 5 NS pdns3.ultradns.org.
bmc.com. 5 NS pdns4.ultradns.org.
bmc.com. 5 NS pdns2.ultradns.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0