Apex domains cannot have CNAME records.
Question and response
Question
dig cbs.com. NS
Answer
cbs.com. 2 NS dns1.p09.nsone.net.
cbs.com. 2 NS dns3.p09.nsone.net.
cbs.com. 2 NS dns2.p09.nsone.net.
cbs.com. 2 NS dns4.p09.nsone.net.
cbs.com. 2 NS ns0004.secondary.cloudflare.com.
cbs.com. 2 NS ns0243.secondary.cloudflare.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0