Apex domains cannot have CNAME records.
Question and response
Question
dig adhost.com. NS
Answer
adhost.com. 4 NS ns3.tierpoint.com.
adhost.com. 4 NS ns1.tierpoint.com.
adhost.com. 4 NS ns2.tierpoint.com.
adhost.com. 4 NS ns4.tierpoint.com.
Authority
The response has no authority records.
Additional
ns1.tierpoint.com. 4 A 216.27.81.251
ns2.tierpoint.com. 4 A 67.216.72.55
ns3.tierpoint.com. 4 A 40.143.16.136
ns4.tierpoint.com. 4 A 173.237.157.193
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0