How do I refactor components and communicate with events?

I'm facing an issue with this Trailhead module - I'm getting the following error message & I really can't see why:

Challenge Not yet complete... Here's what's wrong: The campingListFormHelper isn't resetting the 'newItem' value provider with a Camping_Item__c sObject.

Functionality, my app works exactly as specified - I think there must be some nuisance in whatever way Trailhead checks the functionality that I'm not aware of.

Here's my app:

controller (no helper code for this component):
({
    doInit : function (component, event, helper) {
        var action = component.get("c.getItems");
        action.setCallback(this, function(response) {
            var state = response.getState();
            if (component.isValid() && state == 'SUCCESS') {
                component.set("v.items", response.getReturnValue());
            } else {
                console.log('Failed with state: ' + state);
            }
        });
        $A.enqueueAction(action);
    },
    handleAddItem: function(component, event, helper) {
        var item = event.getParam("item");
        var action = component.get("c.saveItem");
        //json stringify is not needed I think.       
        action.setParams({
                    "item": item
                });
        action.setCallback(this, function(response){
            var state = response.getState();
            if (component.isValid() && state === "SUCCESS") {        
                var items = component.get("v.items");
                items.push(item);
                component.set("v.items",items);
            }
        });
        $A.enqueueAction(action);
    }
})
campingListForm component:

                        default="{'sobjectType':'Camping_Item__c',
                                'Name':'',
                                'Quantity__c':0,
                                'Price__c':0,
                                'Packed__c':false}" />
                    Add Item
                        
      controller:
({
    submitForm : function(component, event, helper) {
        var newItem = component.get("v.newItem");
        var nameField = component.find("itmname");
        var quantityField = component.find("itmquantity");
        var priceField = component.find("itmprice");
        if ($A.util.isEmpty(nameField.get("v.value"))) {
            nameField.set("v.errors", [{message:"Item name can't be blank."}]);   
        } else if ($A.util.isEmpty(quantityField.get("v.value"))) {
            quantityField.set("v.errors", [{message:"Item quantity can't be blank."}]);    
        } else if ($A.util.isEmpty(priceField.get("v.value"))) {
            priceField.set("v.errors", [{message:"Item price can't be blank."}]);    
        } else {

            helper.createItem(component, newItem);
            nameField.set("v.errors", null);
            quantityField.set("v.errors", null);
            priceField.set("v.errors", null);
        }
    }   
})
helper:
({
    createItem : function(component, newItem) {
        // fire event
        var addItem = component.getEvent("addItem");
        addItem.setParams({"item":newItem});
        addItem.fire();
        var newItemStr = "{"sobjectType":"Camping_Item__c","Name":"","Price__c":0,"Quantity__c":0,"Packed__c":false}";
        component.set("v.newItem", JSON.parse(newItemStr));
    }
})

As I said, the application works perfectly and I can't see any reason why it would be failing but it is. I've checked the other answers for similar problems to no avail.