Apex domains cannot have CNAME records.
Question and response
Question
dig ptera.net. NS
Answer
ptera.net. 4 NS ns2.ptera.net.
ptera.net. 4 NS ns1.ptera.net.
Authority
The response has no authority records.
Additional
ns1.ptera.net. 4 A 69.28.32.50
ns2.ptera.net. 4 A 69.28.32.51
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0