Error: Cannot convert value of type 'Int' to expected argument type 'Double?'

@IBOutlet weak var collectionView: UICollectionView!
@IBOutlet weak var totalLabel: UILabel!
@IBOutlet weak var balanceLabel: UILabel!
@IBOutlet weak var quantityLabel: UILabel!
@IBOutlet weak var priceLabel: UILabel!
@IBOutlet weak var quantityStepper: UIStepper!

let vendingMachine: VendingMachine
var currentSelection: VendingSelection?

required init?(coder aDecoder: NSCoder) {
    do {
        let dictionary = try PlistConverter.dictionary(fromFile: "VendingInventory", ofType: "plist")
        let inventory = try InventoryUnarchiver.vendingInventory(fromDictionary: dictionary)
        self.vendingMachine = FoodVendingMachine(inventory: inventory)
    } catch let error {
        fatalError("\(error)")
    }

    super.init(coder: aDecoder)

}

override func viewDidLoad() {
    super.viewDidLoad()
    // Do any additional setup after loading the view, typically from a nib.
    setupCollectionViewCells()

    updateDisplayWith(balance: vendingMachine.amountDeposited, totalPrice: 0, itemPrice: 0, itemQuantity: 1)
}

override func didReceiveMemoryWarning() {
    super.didReceiveMemoryWarning()
    // Dispose of any resources that can be recreated.
}

// MARK: - Setup

func setupCollectionViewCells() {
    let layout = UICollectionViewFlowLayout()
    layout.sectionInset = UIEdgeInsets(top: 0, left: 0, bottom: 20, right: 0)

    let padding: CGFloat = 10
    let itemWidth = screenWidth/3 - padding
    let itemHeight = screenWidth/3 - padding

    layout.itemSize = CGSize(width: itemWidth, height: itemHeight)
    layout.minimumLineSpacing = 10
    layout.minimumInteritemSpacing = 10

    collectionView.collectionViewLayout = layout
}

//MARK: - Vending Machine

@IBAction func purchase() {
    if let currentSelection = currentSelection {
        do {
            try vendingMachine.vend(selection: currentSelection, quantity: Int(quantityStepper.value))
            updateDisplayWith(balance: vendingMachine.amountDeposited, totalPrice: 0.0, itemPrice: 0, itemQuantity: 1)
        } catch VendingMachineError.outOfStock {
            showAlertWith(title: "Out of stock", message: "This item is unavailable. Please make another selection")
        } catch VendingMachineError.invalidSelection {
            showAlertWith(title: "Invalid selection", message: "Please make another selection")
        } catch VendingMachineError.insufficientFunds(let required) {
            let message = "You need ₹\(required) to complete the transaction"
        } catch let error {
            fatalError("\(error)")
        }

        if let indexPath = collectionView.indexPathsForSelectedItems?.first {
            collectionView.deselectItem(at: indexPath, animated: true)
            updateCell(having: indexPath, selected: false)
        }

    } else {
        // FIXME: Alert user no selection
    }
}

func updateDisplayWith(balance: Double? = nil, totalPrice: Double? = nil, itemPrice: Double? = nil, itemQuantity: Double? = nil) {

    if let balanceValue = balance {
        balanceLabel.text = "₹\(balanceValue)"
    }

    if let totalValue = totalPrice {
        totalLabel.text = "₹\(totalValue)"
    }

    if let priceValue = itemPrice {
         priceLabel.text = "₹\(priceValue)"
    }

    if let quantityValue = itemQuantity {
        quantityLabel.text = "\(quantityValue)"
    }
}

func updateTotalPrice(for item: VendingItem) {
    let totalPrice = item.price * quantityStepper.value
    updateDisplayWith(totalPrice: totalPrice)
}

@IBAction func updateValue(_ sender: UIStepper) {
    let quantity = Int(quantityStepper.value)
    updateDisplayWith(itemQuantity: quantity)

    if let currentSelection = currentSelection, let item = vendingMachine.item(forSelection: currentSelection) {
        updateTotalPrice(for: item)
    }
}