diff --git a/chrono/api/views.py b/chrono/api/views.py
index a0a1ec48..ee844ce4 100644
--- a/chrono/api/views.py
+++ b/chrono/api/views.py
@@ -587,6 +587,16 @@ class Fillslots(APIView):
             except ValueError:
                 events = Event.objects.filter(slug__in=slots).order_by('start_datetime')
 
+            if not events.count():
+                return Response(
+                    {
+                        'err': 1,
+                        'err_class': 'unknown event identifiers or slugs',
+                        'err_desc': _('unknown event identifiers or slugs'),
+                    },
+                    status=status.HTTP_400_BAD_REQUEST,
+                )
+
         # search free places. Switch to waiting list if necessary.
         in_waiting_list = False
         for event in events:
diff --git a/tests/test_api.py b/tests/test_api.py
index 23ed41ac..568a31b1 100644
--- a/tests/test_api.py
+++ b/tests/test_api.py
@@ -684,11 +684,9 @@ def test_booking_api_fillslots(app, some_data, user):
     # invalid slots format
     resp = app.post_json('/api/agenda/%s/fillslots/' % agenda.id, params={'slots': 'foobar'}, status=400)
     assert resp.json['err'] == 1
-    assert resp.json['reason'] == 'invalid payload'  # legacy
-    assert resp.json['err_class'] == 'invalid payload'
-    assert resp.json['err_desc'] == 'invalid payload'
-    assert len(resp.json['errors']) == 1
-    assert 'slots' in resp.json['errors']
+    assert resp.json['reason'] == 'unknown event identifiers or slugs'  # legacy
+    assert resp.json['err_class'] == 'unknown event identifiers or slugs'
+    assert resp.json['err_desc'] == 'unknown event identifiers or slugs'
 
     # unknown agendas
     resp = app.post('/api/agenda/foobar/fillslots/', status=404)