diff --git a/arkindex_worker/worker/element.py b/arkindex_worker/worker/element.py
index 32b9694978924e2d479b67b376cb406900ed7c50..f751b19b632416e4b86c695df86db5c2a555e126 100644
--- a/arkindex_worker/worker/element.py
+++ b/arkindex_worker/worker/element.py
@@ -31,7 +31,7 @@ class ElementMixin(object):
 
         if missing_slugs:
             raise MissingTypeError(
-                f'Element type(s) {", ".join(missing_slugs)} were not found in the {corpus["name"]} corpus ({corpus["id"]}).'
+                f'Element type(s) {", ".join(sorted(missing_slugs))} were not found in the {corpus["name"]} corpus ({corpus["id"]}).'
             )
 
         return True
diff --git a/arkindex_worker/worker/entity.py b/arkindex_worker/worker/entity.py
index 9992b5491bf8a6a09c425ec37d297a1ed8ec4bd1..fb8d9f0caae57fc492227746017dc2597af6c9fb 100644
--- a/arkindex_worker/worker/entity.py
+++ b/arkindex_worker/worker/entity.py
@@ -21,7 +21,7 @@ class EntityType(Enum):
 
 class EntityMixin(object):
     def create_entity(
-        self, element, name, type, corpus=None, metas=None, validated=None
+        self, element, name, type, corpus=None, metas=dict(), validated=None
     ):
         """
         Create an entity on the given corpus through API
diff --git a/tests/test_elements_worker/test_elements.py b/tests/test_elements_worker/test_elements.py
index d17a4a5a6d83443d559cbc244618d9b27d40ce44..02c04ca34c83c7020ceec55217f61430f5fd822f 100644
--- a/tests/test_elements_worker/test_elements.py
+++ b/tests/test_elements_worker/test_elements.py
@@ -52,7 +52,7 @@ def test_check_required_types(monkeypatch, tmp_path, mock_elements_worker, respo
         assert worker.check_required_types(corpus_id, "page", "text_line", "act")
     assert (
         str(e.value)
-        == "Element type(s) text_line, act were not found in the Some Corpus corpus (12341234-1234-1234-1234-123412341234)."
+        == "Element type(s) act, text_line were not found in the Some Corpus corpus (12341234-1234-1234-1234-123412341234)."
     )
 
 
diff --git a/tests/test_elements_worker/test_entities.py b/tests/test_elements_worker/test_entities.py
index 301f2a21bf028e0c6e02a85974ece6bc973d8e30..4bc557d4a1d9d31de678507149ebdc92e83111f9 100644
--- a/tests/test_elements_worker/test_entities.py
+++ b/tests/test_elements_worker/test_entities.py
@@ -213,7 +213,7 @@ def test_create_entity(responses, mock_elements_worker):
     assert json.loads(responses.calls[-1].request.body) == {
         "name": "Bob Bob",
         "type": "person",
-        "metas": None,
+        "metas": {},
         "validated": None,
         "corpus": "12341234-1234-1234-1234-123412341234",
         "worker_version": "12341234-1234-1234-1234-123412341234",
@@ -247,7 +247,7 @@ def test_create_entity_with_cache(responses, mock_elements_worker_with_cache):
     assert json.loads(responses.calls[-1].request.body) == {
         "name": "Bob Bob",
         "type": "person",
-        "metas": None,
+        "metas": {},
         "validated": None,
         "corpus": "12341234-1234-1234-1234-123412341234",
         "worker_version": "12341234-1234-1234-1234-123412341234",
@@ -261,7 +261,7 @@ def test_create_entity_with_cache(responses, mock_elements_worker_with_cache):
             type="person",
             name="Bob Bob",
             validated=False,
-            metas=None,
+            metas={},
             worker_version_id=UUID("12341234-1234-1234-1234-123412341234"),
         )
     ]