Apex domains cannot have CNAME records.
Question and response
Question
dig linode.com. NS
Answer
linode.com. 5 NS a1-249.akam.net.
linode.com. 5 NS a12-65.akam.net.
linode.com. 5 NS a16-64.akam.net.
linode.com. 5 NS a3-67.akam.net.
linode.com. 5 NS a7-65.akam.net.
linode.com. 5 NS a4-64.akam.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0