diff --git a/language/functions.xml b/language/functions.xml
index 5a8df8d6dc42..05d9241520cf 100644
--- a/language/functions.xml
+++ b/language/functions.xml
@@ -1258,7 +1258,7 @@ class Cart
     
     public function add($product, $quantity)
     {
-        $this->products[$product] = $quantity;
+        $this->products[$product] = (isset($this->products[$product]) ? $this->products[$product] : 0) + $quantity;
     }
     
     public function getQuantity($product)