Calculate tax on a PO matched invoice - two tax lines created

 Hi Folks

We have a client who has upgraded from 11.5.10 to 12.1.3 and has the STCC tax regime setup (from the upgrade).

Scenario is:

1) Enter a PO and make the tax rate of 0%

2) Match PO to an invoice

3) Calculate tax

4) Two tax lines are created: 

- One at the rate on the PO (0%) and correct

- One at the standard tax rate (20%)

Where is this 20% standard tax defaulting from and how can it be controlled?

Thanks

Rob