diff --git a/spec/unit/matrix-client.spec.js b/spec/unit/matrix-client.spec.js
index a32ee2693..422dc995b 100644
--- a/spec/unit/matrix-client.spec.js
+++ b/spec/unit/matrix-client.spec.js
@@ -134,6 +134,7 @@ describe("MatrixClient", function() {
             "getRoom", "getRooms", "getUser", "getSyncToken", "scrollback",
             "save", "setSyncToken", "storeEvents", "storeRoom", "storeUser",
             "getFilterIdByName", "setFilterIdByName", "getFilter", "storeFilter",
+            "getSyncAccumulator",
         ].reduce((r, k) => { r[k] = expect.createSpy(); return r; }, {});
         client = new MatrixClient({
             baseUrl: "https://my.home.server",
diff --git a/spec/unit/sync-accumulator.spec.js b/spec/unit/sync-accumulator.spec.js
index a7864ca50..b5ca1691c 100644
--- a/spec/unit/sync-accumulator.spec.js
+++ b/spec/unit/sync-accumulator.spec.js
@@ -1,11 +1,27 @@
+/*
+Copyright 2017 Vector Creations 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.
+*/
+
 "use strict";
 import 'source-map-support/register';
-const sdk = require("../..");
-const SyncAccumulator = sdk.SyncAccumulator;
-const utils = require("../test-utils");
-
+import utils from "../test-utils";
+import sdk from "../..";
 import expect from 'expect';
 
+const SyncAccumulator = sdk.SyncAccumulator;
+
 describe("SyncAccumulator", function() {
     let sa;
 
diff --git a/src/client.js b/src/client.js
index e7207e22e..e459209da 100644
--- a/src/client.js
+++ b/src/client.js
@@ -38,7 +38,6 @@ const Filter = require("./filter");
 const SyncApi = require("./sync");
 const MatrixBaseApis = require("./base-apis");
 const MatrixError = httpApi.MatrixError;
-const IndexedDBStore = require("./store/indexeddb").IndexedDBStore;
 
 const SCROLLBACK_DELAY_MS = 3000;
 let CRYPTO_ENABLED = false;
@@ -165,10 +164,7 @@ function MatrixClient(opts) {
         this.olmVersion = Crypto.getOlmVersion();
     }
 
-    // Set up a sync accumulator if we can persist room data
-    if (this.store instanceof IndexedDBStore) {
-        this._syncAccumulator = this.store.getSyncAccumulator();
-    }
+    this._syncAccumulator = this.store.getSyncAccumulator();
 }
 utils.inherits(MatrixClient, EventEmitter);
 utils.extend(MatrixClient.prototype, MatrixBaseApis.prototype);
diff --git a/src/store/memory.js b/src/store/memory.js
index b14b25f7d..d356369ed 100644
--- a/src/store/memory.js
+++ b/src/store/memory.js
@@ -1,5 +1,6 @@
 /*
 Copyright 2015, 2016 OpenMarket Ltd
+Copyright 2017 Vector Creations Ltd
 
 Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.
@@ -279,4 +280,12 @@ module.exports.MatrixInMemoryStore.prototype = {
      * Save does nothing as there is no backing data store.
      */
     save: function() {},
+
+    /**
+     * Returns nothing as this store does not accumulate /sync data.
+     * @return {?SyncAccumulator} null
+     */
+    getSyncAccumulator: function() {
+        return null;
+    },
 };
diff --git a/src/store/stub.js b/src/store/stub.js
index ed9da2ef3..539577f42 100644
--- a/src/store/stub.js
+++ b/src/store/stub.js
@@ -1,5 +1,6 @@
 /*
 Copyright 2015, 2016 OpenMarket Ltd
+Copyright 2017 Vector Creations Ltd
 
 Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.
@@ -184,6 +185,14 @@ StubStore.prototype = {
      * Save does nothing as there is no backing data store.
      */
     save: function() {},
+
+    /**
+     * Returns nothing as this store does not accumulate /sync data.
+     * @return {?SyncAccumulator} null
+     */
+    getSyncAccumulator: function() {
+        return null;
+    },
 };
 
 /** Stub Store class. */
diff --git a/src/sync-accumulator.js b/src/sync-accumulator.js
index ed071c9ff..1a4465555 100644
--- a/src/sync-accumulator.js
+++ b/src/sync-accumulator.js
@@ -13,7 +13,6 @@ 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.
 */
-"use strict";
 
 /**
  * This is an internal module. See {@link SyncAccumulator} for the public class.
diff --git a/src/sync.js b/src/sync.js
index 0b4c5b578..ccf02b026 100644
--- a/src/sync.js
+++ b/src/sync.js
@@ -1,5 +1,6 @@
 /*
 Copyright 2015, 2016 OpenMarket Ltd
+Copyright 2017 Vector Creations Ltd
 
 Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.
@@ -508,9 +509,6 @@ SyncApi.prototype._sync = function(syncOptions) {
         qps.timeout = 0;
     }
 
-    // normal timeout= plus buffer time
-    const clientSideTimeoutMs = this.opts.pollTimeout + BUFFER_PERIOD_MS;
-
     let isCachedResponse = false;
     if (self.opts.syncAccumulator && !syncOptions.hasSyncedBefore) {
         let data = self.opts.syncAccumulator.getJSON();
@@ -520,7 +518,7 @@ SyncApi.prototype._sync = function(syncOptions) {
             debuglog("sync(): not doing HTTP hit, instead returning stored /sync data");
             // We must deep copy the stored data so that the /sync processing code doesn't
             // corrupt the internal state of the sync accumulator (it adds non-clonable keys)
-            data = JSON.parse(JSON.stringify(data));
+            data = utils.deepCopy(data);
             this._currentSyncRequest = q.resolve({
                 next_batch: data.nextBatch,
                 rooms: data.roomsData,
@@ -529,6 +527,9 @@ SyncApi.prototype._sync = function(syncOptions) {
         }
     }
 
+    // normal timeout= plus buffer time
+    const clientSideTimeoutMs = this.opts.pollTimeout + BUFFER_PERIOD_MS;
+
     if (!isCachedResponse) {
         debuglog('Starting sync since=' + syncToken);
         this._currentSyncRequest = client._http.authedRequest(