Apex domains cannot have CNAME records.
Question and response
Question
dig _spf0000007.tangocard.com. NS
Answer
_spf0000007.tangocard.com. 4 NS ns-548.awsdns-04.net.
_spf0000007.tangocard.com. 4 NS ns-1753.awsdns-27.co.uk.
_spf0000007.tangocard.com. 4 NS ns-276.awsdns-34.com.
_spf0000007.tangocard.com. 4 NS ns-1484.awsdns-57.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0