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