Apex domains cannot have CNAME records.
Question and response
Question
dig servercentral.com. NS
Answer
servercentral.com. 5 NS ns1.servercentral.net.
servercentral.com. 5 NS ns2.servercentral.net.
servercentral.com. 5 NS ns3.servercentral.net.
Authority
The response has no authority records.
Additional
ns1.servercentral.net. 5 A 64.202.97.7
ns2.servercentral.net. 5 A 64.202.97.8
ns3.servercentral.net. 5 A 64.202.97.9
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0