In Shopify why am I receiving 'Error: The supplied variables are not according to specification: currency'?

In Shopify why am I receiving 'Error: The supplied variables are not according to specification: currency'?

Unfortunately, Shopify currency conversion with PayFast does not work. The foreign currency amount comes through as ZAR amount because Payfast only transacts in ZAR.

You will need to ensure that the shop currency is set to ZAR.