Swift question

So I'm starting to learn Swift, and I've hit a bit of an issue.

Here's my code

class Account {
    var balance:Float
    var overDraft:Float = 0.00
    var availableBalance:Float {
        get {
            return balance + overDraft
        }
    }
    
    init(amount:Float) {
        self.balance = amount
    }
    
    // Method to debit an amount from the balance
    func debit(amount:Float) {
        if (availableBalance - amount) < 0 {
            println("Insufficient funds")
        } else {
            self.balance = self.balance - amount
        }
    }
    
    // Method to credit an amount to the balance
    func credit(amount:Float) {
        self.balance = self.balance + amount
    }
    
    // Method to adjust the amount of the available overdraft
    func adjustOverdraft(amount:Float) {
        self.overDraft = amount
    }
}

var myAccount = Account(amount: 5.00)
myAccount.adjustOverdraft(10.00)
myAccount.balance
myAccount.availableBalance
myAccount.debit(16.00) // Here insufficient funds should appear
myAccount.availableBalance

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The issue seems to be with the availableBalance property, and obviously I'm doing something wrong in the way I'm returning the availableBalance, because the debit method doesn't work when you don't have sufficient funds, ie. the message "Insufficient funds " doesn't appear.

I've also tried the setting the property using the following

var availableBalance:Float {
        get {
            return self.balance + self.overDraft
        }
    }

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Can someone advise where i'm going wrong?