Apex domains cannot have CNAME records.
Question and response
Question
dig github.io. NS
Answer
github.io. 4 NS ns-1622.awsdns-10.co.uk.
github.io. 4 NS dns3.p05.nsone.net.
github.io. 4 NS ns-692.awsdns-22.net.
github.io. 4 NS ns-1339.awsdns-39.org.
github.io. 4 NS dns1.p05.nsone.net.
github.io. 4 NS dns2.p05.nsone.net.
github.io. 4 NS ns-393.awsdns-49.com.
github.io. 4 NS dns4.p05.nsone.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0