Apex domains cannot have CNAME records.
Question and response
Question
dig _spf0000005.tangocard.com. NS
Answer
_spf0000005.tangocard.com. 5 NS ns-1409.awsdns-48.org.
_spf0000005.tangocard.com. 5 NS ns-616.awsdns-13.net.
_spf0000005.tangocard.com. 5 NS ns-368.awsdns-46.com.
_spf0000005.tangocard.com. 5 NS ns-1840.awsdns-38.co.uk.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0