diff --git a/lobby/tests.py b/lobby/tests.py
index 64e4985..6b156f3 100644
--- a/lobby/tests.py
+++ b/lobby/tests.py
@@ -138,6 +138,17 @@ class LobbyFlowTests(TestCase):
         self.assertEqual(response.json()["locale"], "en")
         self.assertEqual(response.json()["error"], "Session code is required")
 
+    def test_join_error_payload_uses_stable_i18n_contract_keys(self):
+        response = self.client.post(
+            reverse("lobby:join_session"),
+            data={"code": "   ", "nickname": "Luna"},
+            content_type="application/json",
+            HTTP_ACCEPT_LANGUAGE="da",
+        )
+
+        self.assertEqual(response.status_code, 400)
+        self.assertEqual(sorted(response.json().keys()), ["error", "error_code", "locale"])
+
     def test_session_detail_returns_players(self):
         session = GameSession.objects.create(host=self.host, code="LMNO45")
         Player.objects.create(session=session, nickname="Mia", score=7)