Apex domains cannot have CNAME records.
Question and response
Question
dig ns.discovery.com. NS
Answer
ns.discovery.com. 5 NS ns-326.awsdns-40.com.
ns.discovery.com. 5 NS ns-1572.awsdns-04.co.uk.
ns.discovery.com. 5 NS ns-1197.awsdns-21.org.
ns.discovery.com. 5 NS ns-1002.awsdns-61.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0