Apex domains cannot have CNAME records.
Question and response
Question
dig core.fs3d.net. NS
Answer
core.fs3d.net. 5 NS ns2.digitalocean.com.
core.fs3d.net. 5 NS ns1.digitalocean.com.
core.fs3d.net. 5 NS ns3.digitalocean.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0