diff --git a/src/Subject.ts b/src/Subject.ts
index a970cc6aa2..257cb3ab2d 100644
--- a/src/Subject.ts
+++ b/src/Subject.ts
@@ -49,7 +49,7 @@ export class Subject<T> extends Observable<T> implements ISubscription {
     return <any>subject;
   }
 
-  next(value: T) {
+  next(value?: T) {
     if (this.isUnsubscribed) {
       throw new ObjectUnsubscribedError();
     }