diff --git a/openslides/utils/collection.py b/openslides/utils/collection.py
index 661007d00..6993218b2 100644
--- a/openslides/utils/collection.py
+++ b/openslides/utils/collection.py
@@ -506,8 +506,11 @@ def get_model_from_collection_string(collection_string):
                 pass
             else:
                 _models_to_collection_string[get_collection_string()] = model
-
-    return _models_to_collection_string[collection_string]
+    try:
+        model = _models_to_collection_string[collection_string]
+    except KeyError:
+        raise ValueError('Invalid message. A valid collection_string is missing.')
+    return model
 
 
 def get_single_element_cache_key(collection_string, id):
diff --git a/tests/unit/utils/test_collection.py b/tests/unit/utils/test_collection.py
index 065c7683b..9bb292e9b 100644
--- a/tests/unit/utils/test_collection.py
+++ b/tests/unit/utils/test_collection.py
@@ -58,7 +58,7 @@ class TestGetModelFromCollectionString(TestCase):
         self.assertEqual(projector_model, Projector)
 
     def test_unknown_app(self):
-        with self.assertRaises(KeyError):
+        with self.assertRaises(ValueError):
             collection.get_model_from_collection_string('invalid/model')