diff --git a/packages/core-api/src/lib/subjects.ts b/packages/core-api/src/lib/subjects.ts
index 9ebde6ebbc..5239e460af 100644
--- a/packages/core-api/src/lib/subjects.ts
+++ b/packages/core-api/src/lib/subjects.ts
@@ -53,6 +53,10 @@ export class PublishSubject<T>
     ZenObservable.SubscriptionObserver<T>
   >();
 
+  [Symbol.observable]() {
+    return this;
+  }
+
   get closed() {
     return this.isClosed;
   }
@@ -148,6 +152,10 @@ export class BehaviorSubject<T>
     ZenObservable.SubscriptionObserver<T>
   >();
 
+  [Symbol.observable]() {
+    return this;
+  }
+
   get closed() {
     return this.isClosed;
   }
diff --git a/packages/core-api/src/types.ts b/packages/core-api/src/types.ts
index 13a8a72570..8782280be0 100644
--- a/packages/core-api/src/types.ts
+++ b/packages/core-api/src/types.ts
@@ -39,7 +39,7 @@ export type Subscription = {
   /**
    * Value indicating whether the subscription is closed.
    */
-  readonly closed: Boolean;
+  readonly closed: boolean;
 };
 
 /**
@@ -51,12 +51,14 @@ export type Subscription = {
  * using many different observable implementations, such as zen-observable or RxJS 5.
  */
 export type Observable<T> = {
+  [Symbol.observable](): Observable<T>;
+
   /**
    * Subscribes to this observable to start receiving new values.
    */
   subscribe(observer: Observer<T>): Subscription;
   subscribe(
-    onNext: (value: T) => void,
+    onNext?: (value: T) => void,
     onError?: (error: Error) => void,
     onComplete?: () => void,
   ): Subscription;
diff --git a/packages/test-utils/src/testUtils/apis/ErrorApi/MockErrorApi.ts b/packages/test-utils/src/testUtils/apis/ErrorApi/MockErrorApi.ts
index 7bf50314d0..b90646201a 100644
--- a/packages/test-utils/src/testUtils/apis/ErrorApi/MockErrorApi.ts
+++ b/packages/test-utils/src/testUtils/apis/ErrorApi/MockErrorApi.ts
@@ -32,6 +32,10 @@ type Waiter = {
 
 const nullObservable = {
   subscribe: () => ({ unsubscribe: () => {}, closed: true }),
+
+  [Symbol.observable]() {
+    return this;
+  },
 };
 
 export class MockErrorApi implements ErrorApi {