diff --git a/packages/idb-bridge/src/MemoryBackend.test.ts b/packages/idb-bridge/src/MemoryBackend.test.ts
index 2e322dfa6..c21c2d064 100644
--- a/packages/idb-bridge/src/MemoryBackend.test.ts
+++ b/packages/idb-bridge/src/MemoryBackend.test.ts
@@ -214,6 +214,23 @@ test("Spec: Example 1 Part 3", async t => {
   cursor = request6.result;
   t.is(cursor, null);
 
+
+  const request7 = index5.openCursor(null, "prevunique");
+  await promiseFromRequest(request7);
+  cursor = request7.result;
+  t.is(cursor.value.author, "Fred");
+  t.is(cursor.value.isbn, 234567);
+  cursor.continue();
+
+  await promiseFromRequest(request7);
+  cursor = request7.result;
+  t.is(cursor.value.author, "Barney");
+  cursor.continue();
+
+  await promiseFromRequest(request7);
+  cursor = request7.result;
+  t.is(cursor, null);
+
   db.close();
 
   t.pass();
diff --git a/packages/idb-bridge/src/MemoryBackend.ts b/packages/idb-bridge/src/MemoryBackend.ts
index 786fa4cf6..694daacfa 100644
--- a/packages/idb-bridge/src/MemoryBackend.ts
+++ b/packages/idb-bridge/src/MemoryBackend.ts
@@ -640,7 +640,16 @@ export class MemoryBackend implements Backend {
     objectStoreProperties.indexes.push(indexName);
     schema.indexes[indexName] = indexProperties;
 
-    // FIXME: build index from existing object store!
+    const objectStore = myConn.objectStoreMap[objectStoreName];
+    if (!objectStore) {
+      throw Error("object store does not exist");
+    }
+
+    const storeData = objectStore.modifiedData || objectStore.originalData;
+
+    storeData.forEach((v, k) => {
+      this.insertIntoIndex(newIndex, k, v.value, indexProperties);
+    });
   }
 
   async deleteRecord(
diff --git a/packages/idb-bridge/src/tree/interfaces.ts b/packages/idb-bridge/src/tree/interfaces.ts
index c708c20b5..6bd0cdf58 100644
--- a/packages/idb-bridge/src/tree/interfaces.ts
+++ b/packages/idb-bridge/src/tree/interfaces.ts
@@ -53,7 +53,7 @@ export interface IMapSource<K=any, V=any> extends ISetSource<K>
   /** Calls callbackFn once for each key-value pair present in the map object.
    *  The ES6 Map class sends the value to the callback before the key, so
    *  this interface must do likewise. */
-  forEach(callbackFn: (v:V, k:K, map:IMapSource<K,V>) => void, thisArg: any): void;
+  forEach(callbackFn: (v:V, k:K, map:IMapSource<K,V>) => void, thisArg?: any): void;
   
   /** Returns an iterator that provides all key-value pairs from the collection (as arrays of length 2). */
   entries(): IterableIterator<[K,V]>;