diff --git a/cart/cart.py b/cart/cart.py
index ff08dc1..769a44b 100644
--- a/cart/cart.py
+++ b/cart/cart.py
@@ -66,6 +66,8 @@ def update(self, product, quantity, unit_price=None):
                 cart=self.cart,
                 product=product,
             )
+            item.quantity += quantity
+            item.save()
         except models.Item.DoesNotExist:
             raise ItemDoesNotExist