diff --git a/lib/store/webstorage.js b/lib/store/webstorage.js
index 506215bbf..261a0eda6 100644
--- a/lib/store/webstorage.js
+++ b/lib/store/webstorage.js
@@ -111,30 +111,32 @@
  * @module store/webstorage
  */
 
+var utils = require("../utils");
+
 /**
  * Construct a web storage store, capable of storing rooms and users.
  * @constructor
- * @param {string} kind The kind of storage. Either 'local' or 'session'.
+ * @param {WebStorage} store A web storage implementation, e.g.
+ * 'window.localStorage' or 'window.sessionStorage' or a custom implementation.
  * @param {integer} batchSize The number of events to store per key/value (room
  * scoped). Use -1 to store all events for a room under one key/value.
- * @throws if the global 'localStorage' does not exist and kind='local', or the
- * global 'sessionStorage' does not exist and kind='session', or kind is not
- * 'local' or 'session'.
+ * @throws if the supplied 'store' does not meet the Storage interface of the
+ * WebStorage API.
  */
-function WebStorageStore(kind, batchSize) {
-    if (["local", "session"].indexOf(kind) === -1) {
-        throw new Error("Invalid kind: " + kind);
-    }
-    if (kind === "local" && !global.localStorage ||
-            kind === "session" && !global.sessionStorage) {
-        throw new Error("Global for " + kind + " storage not found.");
-    }
-    var stores = {
-        local: global.localStorage,
-        session: global.sessionStorage
-    };
-    this.store = stores[kind];
+function WebStorageStore(store, batchSize) {
+    this.store = store;
     this.batchSize = batchSize;
+    if (!utils.isFunction(store.getItem) || !utils.isFunction(store.setItem) ||
+            !utils.isFunction(store.removeItem) || !utils.isFunction(store.key)) {
+        throw new Error(
+            "Supplied store does not meet the WebStorage API interface"
+        );
+    }
+    if (!parseInt(store.length) && store.length !== 0) {
+        throw new Error(
+            "Supplied store does not meet the WebStorage API interface (length)"
+        );
+    }
 }
 
 WebStorageStore.prototype = {