Apex domains cannot have CNAME records.
Question and response
Question
dig servernap.com. NS
Answer
servernap.com. 5 NS ns2.servernap.com.
servernap.com. 5 NS ns4.servernap.com.
servernap.com. 5 NS ns1.servernap.com.
servernap.com. 5 NS ns3.servernap.com.
Authority
The response has no authority records.
Additional
ns1.servernap.com. 5 A 69.65.17.101
ns2.servernap.com. 5 A 69.65.16.102
ns3.servernap.com. 5 A 69.65.17.101
ns4.servernap.com. 5 A 69.65.16.102
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0