diff --git a/spec/util/isObservable-spec.ts b/spec/util/isObservable-spec.ts
index 91d7d2ae0a..24fb6f69dc 100644
--- a/spec/util/isObservable-spec.ts
+++ b/spec/util/isObservable-spec.ts
@@ -23,4 +23,13 @@ describe('isObservable', () => {
 
     expect(isObservable(o)).to.be.false;
   });
+
+  it('should return false for null', () => {
+    expect(isObservable(null)).to.be.false;
+  });
+
+  it('should return false for a number', () => {
+    expect(isObservable(1)).to.be.false;
+  });
+
 });
diff --git a/src/internal/util/isObservable.ts b/src/internal/util/isObservable.ts
index 2bda600928..f3df52485b 100644
--- a/src/internal/util/isObservable.ts
+++ b/src/internal/util/isObservable.ts
@@ -6,5 +6,5 @@ import { ObservableInput } from '../types';
  * @param obj the object to test
  */
 export function isObservable<T>(obj: any): obj is Observable<T> {
-  return obj && obj instanceof Observable || (typeof obj.lift === 'function' && typeof obj.subscribe === 'function');
+  return !!obj && (obj instanceof Observable || (typeof obj.lift === 'function' && typeof obj.subscribe === 'function'));
 }