Apex domains cannot have CNAME records.
Question and response
Question
dig _spf0000000.tangocard.com. NS
Answer
_spf0000000.tangocard.com. 5 NS ns-891.awsdns-47.net.
_spf0000000.tangocard.com. 5 NS ns-1502.awsdns-59.org.
_spf0000000.tangocard.com. 5 NS ns-2012.awsdns-59.co.uk.
_spf0000000.tangocard.com. 5 NS ns-172.awsdns-21.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0