Apex domains cannot have CNAME records.
Question and response
Question
dig github.com. NS
Answer
github.com. 4 NS ns-421.awsdns-52.com.
github.com. 4 NS dns2.p08.nsone.net.
github.com. 4 NS ns-1283.awsdns-32.org.
github.com. 4 NS dns4.p08.nsone.net.
github.com. 4 NS dns1.p08.nsone.net.
github.com. 4 NS dns3.p08.nsone.net.
github.com. 4 NS ns-520.awsdns-01.net.
github.com. 4 NS ns-1707.awsdns-21.co.uk.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0