diff --git a/src/pretix/presale/views/cart.py b/src/pretix/presale/views/cart.py
index 01a0dbfca8..cb2533c8c7 100644
--- a/src/pretix/presale/views/cart.py
+++ b/src/pretix/presale/views/cart.py
@@ -543,7 +543,10 @@ class CartExtendReservation(EventViewMixin, CartActionMixin, AsyncAction, View):
     known_errortypes = ['CartError']
 
     def _ajax_response_data(self, value):
-        return value or {}
+        if isinstance(value, dict):
+            return value
+        else:
+            return {}
 
     def get_success_message(self, value):
         if value['success'] > 0:
diff --git a/src/pretix/static/pretixbase/js/asynctask.js b/src/pretix/static/pretixbase/js/asynctask.js
index 9e90663703..7bbd72fa68 100644
--- a/src/pretix/static/pretixbase/js/asynctask.js
+++ b/src/pretix/static/pretixbase/js/asynctask.js
@@ -41,7 +41,7 @@ function async_task_schedule_check(context, timeout) {
 
 function async_task_on_success(data) {
     "use strict";
-    if ((async_task_is_download || async_task_dont_redirect) && data.success) {
+    if ((async_task_is_download && data.success) || async_task_dont_redirect) {
         waitingDialog.hide();
         if (location.href.indexOf("async_id") !== -1) {
             history.replaceState({}, "pretix", async_task_old_url);
diff --git a/src/pretix/static/pretixpresale/js/ui/cart.js b/src/pretix/static/pretixpresale/js/ui/cart.js
index 819e819d5c..b34b298ee4 100644
--- a/src/pretix/static/pretixpresale/js/ui/cart.js
+++ b/src/pretix/static/pretixpresale/js/ui/cart.js
@@ -101,8 +101,11 @@ $(function () {
         cart.init();
     }
 
-    $("#cart-extend-form").on("pretix:async-task-success", function(e, data, x, y, z) {
-        cart.set_deadline(data.expiry, data.max_expiry_extend);
+    $("#cart-extend-form").on("pretix:async-task-success", function(e, data) {
+        if (data.success)
+            cart.set_deadline(data.expiry, data.max_expiry_extend);
+        else
+            alert(data.message);
     });
 
     $("#cart-extend-modal button").click(function() {