diff --git a/src/pretix/presale/views/cart.py b/src/pretix/presale/views/cart.py
index 7f69da6127..01a0dbfca8 100644
--- a/src/pretix/presale/views/cart.py
+++ b/src/pretix/presale/views/cart.py
@@ -543,7 +543,7 @@ class CartExtendReservation(EventViewMixin, CartActionMixin, AsyncAction, View):
     known_errortypes = ['CartError']
 
     def _ajax_response_data(self, value):
-        return value
+        return value or {}
 
     def get_success_message(self, value):
         if value['success'] > 0: