Calculation results in empty cells

Hello support,

I followed the instructions of this:
https://docs.algolplus.com/algol_order_export/add-calculated-field-for-product/

and added a new field.
This PHP code for the calculation:

******
add_filter(‘woe_get_order_product_value_btwverzendcalc’, function ($value, $order, $item, $product,$item_meta) {
return $item[‘order_shipping_tax’]/$item[‘order_shipping’];
}, 10, 5);
*****

is resulting to empty cells.

The added field: I’ve tried string, number and money for the format and all result in empty cells. The debug mode does not give me any clue to what could be wrong in this line.

What am I missing?