aldo-roman · Sep 8, 2017 · Jan 16, 2018 · Jul 27, 2018 · Aug 1, 2018 · Aug 2, 2018
diff --git a/packages/core/src/util/lang.ts b/packages/core/src/util/lang.ts
@@ -21,6 +21,7 @@ export function isPromise(obj: any): obj is Promise<any> {
  * Determine if the argument is an Observable
  */
 export function isObservable(obj: any | Observable<any>): obj is Observable<any> {
-  // TODO: use Symbol.observable when https://github.com/ReactiveX/rxjs/issues/2415 will be resolved
+  // TODO: use isObservable once we update pass rxjs 6.1
+  // https://github.com/ReactiveX/rxjs/blob/master/CHANGELOG.md#610-2018-05-03
   return !!obj && typeof obj.subscribe === 'function';
 }
diff --git a/packages/core/src/view/provider.ts b/packages/core/src/view/provider.ts
@@ -13,6 +13,7 @@ import {TemplateRef} from '../linker/template_ref';
 import {ViewContainerRef} from '../linker/view_container_ref';
 import {Renderer as RendererV1, Renderer2} from '../render/api';
 import {stringify} from '../util';
+import {isObservable} from '../util/lang';
 
 import {createChangeDetectorRef, createInjector, createRendererV1} from './refs';
 import {BindingDef, BindingFlags, DepDef, DepFlags, NodeDef, NodeFlags, OutputDef, OutputType, ProviderData, QueryValueType, Services, ViewData, ViewFlags, ViewState, asElementData, asProviderData, shouldCallLifecycleInitHook} from './types';
@@ -137,9 +138,15 @@ export function createDirectiveInstance(view: ViewData, def: NodeDef): any {
   if (def.outputs.length) {
     for (let i = 0; i < def.outputs.length; i++) {
       const output = def.outputs[i];
-      const subscription = instance[output.propName !].subscribe(
-          eventHandlerClosure(view, def.parent !.nodeIndex, output.eventName));
-      view.disposables ![def.outputIndex + i] = subscription.unsubscribe.bind(subscription);
+      const outputObservable = instance[output.propName !];
+      if (isObservable(outputObservable)) {
+        const subscription = outputObservable.subscribe(
+            eventHandlerClosure(view, def.parent !.nodeIndex, output.eventName));
+        view.disposables ![def.outputIndex + i] = subscription.unsubscribe.bind(subscription);
+      } else {
+        throw new Error(
+            `@Output ${output.propName} not initialized in '${instance.constructor.name}'.`);
+      }
     }
   }
   return instance;

diff --git a/packages/core/test/bundling/hello_world_r2/bundle.golden_symbols.json b/packages/core/test/bundling/hello_world_r2/bundle.golden_symbols.json
@@ -3170,6 +3170,9 @@
   {
     "name": "isObservable"
   },
+  {
+    "name": "isObservable$1"
+  },
   {
     "name": "isPrefixEnd"
   },
@@ -3839,4 +3842,4 @@
   {
     "name": "wtfLeave"
   }
-]
+]
diff --git a/packages/core/test/linker/integration_spec.ts b/packages/core/test/linker/integration_spec.ts
@@ -346,6 +346,21 @@ function declareTests({useJit}: {useJit: boolean}) {
         expect(tc.injector.get(EventDir)).not.toBeNull();
       });
 
+      it('should display correct error message for uninitialized @Output', () => {
+        @Component({selector: 'my-uninitialized-output', template: '<p>It works!</p>'})
+        class UninitializedOutputComp {
+          @Output() customEvent !: EventEmitter<any>;
+        }
+
+        const template =
+            '<my-uninitialized-output (customEvent)="doNothing()"></my-uninitialized-output>';
+        TestBed.overrideComponent(MyComp, {set: {template}});
+
+        TestBed.configureTestingModule({declarations: [MyComp, UninitializedOutputComp]});
+        expect(() => TestBed.createComponent(MyComp))
+            .toThrowError('@Output customEvent not initialized in \'UninitializedOutputComp\'.');
+      });
+
       it('should read directives metadata from their binding token', () => {
         TestBed.configureTestingModule({declarations: [MyComp, PrivateImpl, NeedsPublicApi]});
         const template = '<div public-api><div needs-public-api></div></div>';