Tax Variance Hold

We have a tax rate with default Tax rate percentage = 0. This rate is set to allow Ad Hoc Rate. This way the AP Invoice entry users can update the tax rate at the invoice line level (In Tax details page) to a desired % in certain cases. We also have Tax Tolerance set to 10%. That said, all the Invoices that use this rate code are being put on 'Tax Variance' Hold. Is it because the original rate is setup as zero? If so, can you suggest setup changes that need to be done to ensure that the invoices