Plugin throws an error on currency change flag

When we change the currency, an arg is passed in the URL as ?currency=USD.

This always causes the following PHP error in the shipping plugin code: call_user_func(): Argument #1 ($callback) must be a valid callback, class Woo_Conditional_Shipping_Filters does not have a method “filter_category”

public?/?content?/?plugins?/?conditional-shipping-for-woocommerce?/?includes?/?class-woo-conditional-shipping-ruleset.php?:?250

Adding debug, when this fails, the $callable variable is:

array:2 [▼<samp> 0 => “Woo_Conditional_Shipping_Filters” 1 => “filter_products” </samp>]

Any help you can offer would be hugely appreciated