Cannot Apply Tax to Foreign AR Invoice

I have a requirement for Singapore to apply a tax code "ZERO RATE" to an AR Invoice when the customer is not from Singapore (i.e. foreign).  Even when I turn off all tax rules, no tax is applied. I have no problems applying tax when the AP supplier is foreign or to other local AR customers. The only issue is with AR customers who are foreign.  I have even tried creating a rule to force the tax application (Tax Rate Rule: If BILL TO COUNTRY <> SINGAPORE, RESULT = ZERO RATE tax code).  Any thoughts on what the issue is?

Thanks,