Apex domains cannot have CNAME records.
Question and response
Question
dig _spf0000001.tangocard.com. NS
Answer
_spf0000001.tangocard.com. 5 NS ns-204.awsdns-25.com.
_spf0000001.tangocard.com. 5 NS ns-1549.awsdns-01.co.uk.
_spf0000001.tangocard.com. 5 NS ns-1322.awsdns-37.org.
_spf0000001.tangocard.com. 5 NS ns-578.awsdns-08.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0