diff --git a/synapse/handlers/device.py b/synapse/handlers/device.py
index f4bf159bb..8d630c6b1 100644
--- a/synapse/handlers/device.py
+++ b/synapse/handlers/device.py
@@ -29,7 +29,7 @@ class DeviceHandler(BaseHandler):
 
     @defer.inlineCallbacks
     def check_device_registered(self, user_id, device_id,
-                                initial_device_display_name):
+                                initial_device_display_name=None):
         """
         If the given device has not been registered, register it with the
         supplied display name.
diff --git a/synapse/rest/client/v2_alpha/keys.py b/synapse/rest/client/v2_alpha/keys.py
index 705a0b6c1..c5ff16adf 100644
--- a/synapse/rest/client/v2_alpha/keys.py
+++ b/synapse/rest/client/v2_alpha/keys.py
@@ -130,9 +130,7 @@ class KeyUploadServlet(RestServlet):
         # old access_token without an associated device_id. Either way, we
         # need to double-check the device is registered to avoid ending up with
         # keys without a corresponding device.
-        self.device_handler.check_device_registered(
-            user_id, device_id, "unknown device"
-        )
+        self.device_handler.check_device_registered(user_id, device_id)
 
         result = yield self.store.count_e2e_one_time_keys(user_id, device_id)
         defer.returnValue((200, {"one_time_key_counts": result}))
diff --git a/synapse/storage/schema/delta/33/devices_for_e2e_keys.sql b/synapse/storage/schema/delta/33/devices_for_e2e_keys.sql
index 140f2b63e..aa4a3b9f2 100644
--- a/synapse/storage/schema/delta/33/devices_for_e2e_keys.sql
+++ b/synapse/storage/schema/delta/33/devices_for_e2e_keys.sql
@@ -16,4 +16,4 @@
 -- make sure that we have a device record for each set of E2E keys, so that the
 -- user can delete them if they like.
 INSERT INTO devices
-    SELECT user_id, device_id, 'unknown device' FROM e2e_device_keys_json;
+    SELECT user_id, device_id, NULL FROM e2e_device_keys_json;
diff --git a/synapse/storage/schema/delta/33/devices_for_e2e_keys_clear_unknown_device.sql b/synapse/storage/schema/delta/33/devices_for_e2e_keys_clear_unknown_device.sql
new file mode 100644
index 000000000..667157339
--- /dev/null
+++ b/synapse/storage/schema/delta/33/devices_for_e2e_keys_clear_unknown_device.sql
@@ -0,0 +1,20 @@
+/* Copyright 2016 OpenMarket Ltd
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+-- a previous version of the "devices_for_e2e_keys" delta set all the device
+-- names to "unknown device". This wasn't terribly helpful
+UPDATE devices
+    SET display_name = NULL
+    WHERE display_name = 'unknown device';