Apex domains cannot have CNAME records.
Question and response
Question
dig fastly.net. NS
Answer
fastly.net. 5 NS ns3.fastly.net.
fastly.net. 5 NS ns2.fastly.net.
fastly.net. 5 NS ns1.fastly.net.
fastly.net. 5 NS ns4.fastly.net.
Authority
The response has no authority records.
Additional
ns1.fastly.net. 5 A 23.235.32.32
ns2.fastly.net. 5 A 104.156.80.32
ns3.fastly.net. 5 A 23.235.36.32
ns4.fastly.net. 5 A 104.156.84.32
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0