I am creating a GTM variable that amends a custom metric in our data layer (standard enhanced e-commerce), that is then passed through to a Google Analytics Event. The full code looks like the below

function() {
var ecom = JSON.parse(JSON.stringify({{dlv_ecommerce}}));
var orderItems = {{dlv_purchase_ecommerce.purchase.products}};
var convertFrom = {{dlv_purchase_ecommerce.currencyCode}};
var rates = {{dlv_rates}};
var eventItem = orderItems[i].category;
var marginPrice;
if(eventItem=="Red Hoodie"){
    marginPrice=orderItems[i].price * 0.10;
}
if(eventItem=="Blue T-Shirt"){
    marginPrice=orderItems[i].price * 0.05;
}
else{
    marginPrice=orderItems[i].metric5;
}
var n = orderItems.length;
lineItems = [];
for (i = 0; i < n; i++) {
  lineItems.push({
    "name": orderItems[i].name,
    "id": orderItems[i].id,
    "price": orderItems[i].price,
    "brand": orderItems[i].brand,
    "category": orderItems[i].category,
    "quantity": orderItems[i].quantity,
    "dimension2": orderItems[i].dimension2,
    "dimension8": orderItems[i].dimension8,
    "dimension9": orderItems[i].dimension9,
    "dimension10": orderItems[i].dimension10,
    "dimension11": orderItems[i].dimension11,
    "metric1": orderItems[i].metric1, 
    "metric2": orderItems[i].metric2, 
    "metric3": orderItems[i].metric3, 
    "metric4": orderItems[i].metric4,
    "metric5": marginPrice / rates[convertFrom], 

  });
};

var ecomAddProducts = {
  "products": lineItems,
};
return {'ecommerce' : {'purchase' : {'products' : ecomAddProducts}}};

}

Everything here works, apart from metric 5, and so i assume also this section (If I do "metric5": orderItems[i].metric5 / rates[convertFrom] It works fine, which is why i'm guessing it's this section)

var eventItem = orderItems[i].category;
var marginPrice;
if(eventItem=="Red Hoodie"){
    marginPrice=orderItems[i].price * 0.10;
}
if(eventItem=="Blue T-Shirt"){
    marginPrice=orderItems[i].price * 0.05;
}
else{
    marginPrice=orderItems[i].metric5;
}

As soon as I add these sections in I get an undefined result in GTM preview mode. Can anyone spot what is causing the issue?