Default tax rate is not firing for NT province of Canada

Hi All,

Appreciate if anyone can respond asap.I have default tax rate which is enabled from 01-Jan-2008 with 5%. When i  am creating a SO my ship_to location is having province 'NT' and Bill_to: 'BC' province. There is tax applicability rule for 'NT' province and there is no tax rate rule.Ideally the default tax rate should be picked.But there are few tax rate rules under DetermineTaxRate in TaxRules with different provinces.when creating SO i m getting error msg: "A valid tax rate does not exist  for Tax: CA GST AR, Tax Status: CA GST AR STD and tax_determination date".