Apex domains cannot have CNAME records.
Question and response
Question
dig cas3.psek.airpush.com. NS
Answer
cas3.psek.airpush.com. 5 NS ns2.afternic.com.
cas3.psek.airpush.com. 5 NS ns1.afternic.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0