Getting error making in app payment

I am trying to do an in app purchase using StoreKit. I can successfully get the SKProduct in stance, but when I pass it into this method to actually make the paytment:

private void MakePayment (SKProduct product)
{
   SKPayment payment = SKPayment.PaymentWithProduct (product);
   SKPaymentQueue.DefaultQueue.AddPayment (payment);
}

I get an exception in the AddPayment method that says this:

Failed to marshal the Objective-C object 0x14607110 (type: SKPaymentTransaction). Could not find an existing managed instance for this object, nor was it possible to create a new managed instance (because the type 'MonoTouch.StoreKit.SKPaymentTransaction[]' does not have a constructor that takes one IntPtr argument).

Any ideas what I am doing wrong?