1 files changed, 18 insertions, 0 deletions
diff --git a/tests/handlers/test_device.py b/tests/handlers/test_device.py
index a3aa0a1cf2..62b47f6574 100644
--- a/tests/handlers/test_device.py
+++ b/tests/handlers/test_device.py
@@ -160,6 +160,24 @@ class DeviceTestCase(unittest.HomeserverTestCase):
         res = self.get_success(self.handler.get_device(user1, "abc"))
         self.assertEqual(res["display_name"], "new display")
 
+    def test_update_device_too_long_display_name(self):
+        """Update a device with a display name that is invalid (too long)."""
+        self._record_users()
+
+        # Request to update a device display name with a new value that is longer than allowed.
+        update = {
+            "display_name": "a"
+            * (synapse.handlers.device.MAX_DEVICE_DISPLAY_NAME_LEN + 1)
+        }
+        self.get_failure(
+            self.handler.update_device(user1, "abc", update),
+            synapse.api.errors.SynapseError,
+        )
+
+        # Ensure the display name was not updated.
+        res = self.get_success(self.handler.get_device(user1, "abc"))
+        self.assertEqual(res["display_name"], "display 2")
+
     def test_update_unknown_device(self):
         update = {"display_name": "new_display"}
         res = self.handler.update_device("user_id", "unknown_device_id", update)