diff --git a/CHANGELOG.md b/CHANGELOG.md
index 5abfb3a..c2ffd48 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,10 @@
 # Changelog
 
+## Unreleased
+
+### Fixed
+- Fixed an error that occurred when using `craft.reorder.unavailableLineItems()`
+
 ## 2.0.0-beta.1 - 2022-03-17
 
 ### Added
diff --git a/src/Variable.php b/src/Variable.php
index 229bcb5..97466c5 100644
--- a/src/Variable.php
+++ b/src/Variable.php
@@ -2,8 +2,9 @@
 
 namespace spicyweb\reorder;
 
+use Craft;
 use craft\commerce\elements\Order;
-
+use craft\commerce\Plugin as Commerce;
 use spicyweb\reorder\Plugin as ReOrder;
 
 /**
@@ -19,11 +20,24 @@ class Variable
      * Checks an order's line items and returns the unavailable items along with why they're unavailable.
      *
      * @param Order $order The order to check.
-     * @param int $cartId A cart ID, to check for the quantity of items already in the user's cart.
+     * @param Order|int|null $cart A cart, or cart ID, to check for the quantity of items already in the user's cart.
      * return array The line items that are unavailable and why.
      */
-    public function unavailableLineItems(Order $order, int $cartId = null): array
+    public function unavailableLineItems(Order $order, Order|int|null $cart = null): array
     {
-        return ReOrder::$plugin->methods->getUnavailableLineItems($order, $cartId);
+        if (is_int($cart)) {
+            $cart = Commerce::getInstance()->getOrders()->getOrderById($cart);
+        }
+
+        // Make sure the order and cart belong to the current user, otherwise tell them nothing
+        $currentUser = Craft::$app->getUser();
+        $orderUser = $order->getCustomer();
+        $cartUser = $cart !== null ? $cart->getCustomer() : null;
+
+        if ((int)$currentUser->id !== (int)$orderUser->id || $cart !== null && (int)$currentUser->id !== (int)$cartUser->id) {
+            return [];
+        }
+
+        return ReOrder::$plugin->methods->getUnavailableLineItems($order, $cart);
     }
 }