diff --git a/src/pretix/base/services/orders.py b/src/pretix/base/services/orders.py
index 634033b151..2e39424237 100644
--- a/src/pretix/base/services/orders.py
+++ b/src/pretix/base/services/orders.py
@@ -457,7 +457,7 @@ class OrderChangeManager:
             if diff <= 0:
                 continue
             avail = quota.availability()
-            if avail[0] != Quota.AVAILABILITY_OK or avail[1] < diff:
+            if avail[0] != Quota.AVAILABILITY_OK or (avail[1] is not None and avail[1] < diff):
                 raise OrderError(self.error_messages['quota'].format(name=quota.name))
 
     def _check_free_to_paid(self):
diff --git a/src/tests/base/test_orders.py b/src/tests/base/test_orders.py
index 73ce2f36cd..dbc8c6023f 100644
--- a/src/tests/base/test_orders.py
+++ b/src/tests/base/test_orders.py
@@ -188,6 +188,14 @@ class OrderChangeManagerTests(TestCase):
     def test_empty(self):
         self.ocm.commit()
 
+    def test_quota_unlimited(self):
+        q = self.event.quotas.create(name='Test', size=None)
+        q.items.add(self.shirt)
+        self.ocm.change_item(self.op1, self.shirt, None)
+        self.ocm.commit()
+        self.op1.refresh_from_db()
+        assert self.op1.item == self.shirt
+
     def test_quota_full(self):
         q = self.event.quotas.create(name='Test', size=0)
         q.items.add(self.shirt)