Apex domains cannot have CNAME records.
Question and response
Question
dig response.pure360.com. NS
Answer
response.pure360.com. 5 NS ns1.dnssub.com.
response.pure360.com. 5 NS ns2.dnssub.com.
response.pure360.com. 5 NS ns0.dnssub.com.
Authority
The response has no authority records.
Additional
ns0.dnssub.com. 5 A 52.209.229.125
ns1.dnssub.com. 5 A 52.51.194.135
ns2.dnssub.com. 5 A 52.211.198.14
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0