Cupon code - free product and subscription time cupon

We have a setup where we sell cupon codes with a amount of time. You can buy 1 year or 3 months. Payment is made with cash at conferences and at a webshop. (This is manual done and we will continue with this.)

I have setup a fixed plan with the price: 0, expiration time: 1 day.

I made a cupon code, type: "Subscription Extension", extend time: 1 year.

When I try to purchase this plan with the cupon code, i get the following error: "Maximum allowed discount crossed, so only allowable discount applied"

The troubles is in the last if statement, because: $existingDiscounts = "0.00" and $allowedAmount = float(0).

public function checkForUpperLimit(&$newModifier, $invoice)

{

	$subTotal = $invoice->getSubtotal();

	$discountable = $invoice->getDiscountable();

	$actualTotal = $subTotal + $discountable;



	$maxDiscount = (float) $this->config->get('allowedMaxPercentDiscount');

	$allowedAmount = ($maxDiscount * $actualTotal) / 100;



	$existingDiscounts = $invoice->getDiscount();



	// If existing discount is already greater or equal to max allowed

	if ($existingDiscounts >= $allowedAmount) {

		return false;

	}

[...]

Discount is set to 100% in the global settings.

Is my approach wrong or could we make a fix?

Thanks in advance!