diff --git a/src/InnerSubscriber.ts b/src/InnerSubscriber.ts
index 6fe5a1bad6..91320ae074 100644
--- a/src/InnerSubscriber.ts
+++ b/src/InnerSubscriber.ts
@@ -4,10 +4,14 @@ import {OuterSubscriber} from './OuterSubscriber';
 export class InnerSubscriber<T, R> extends Subscriber<R> {
   private index: number = 0;
 
-  constructor(private parent: OuterSubscriber<T, R>, private outerValue: T, private outerIndex: number) {
+  constructor(private parent: OuterSubscriber<T, R>, private outerValue: T, private _outerIndex: number) {
     super();
   }
 
+  get outerIndex(): number {
+    return this._outerIndex;
+  }
+
   protected _next(value: R): void {
     this.parent.notifyNext(this.outerValue, value, this.outerIndex, this.index++, this);
   }