diff --git a/src/classic/class/ReactClass.js b/src/classic/class/ReactClass.js
index 68806d0006..052545f6c7 100644
--- a/src/classic/class/ReactClass.js
+++ b/src/classic/class/ReactClass.js
@@ -761,10 +761,11 @@ var ReactClassMixin = {
       }
     }
     var internalInstance = ReactInstanceMap.get(this);
-    return (
-      internalInstance &&
-      internalInstance !== ReactLifeCycle.currentlyMountingInstance
-    );
+    if (internalInstance) {
+      return internalInstance !== ReactLifeCycle.currentlyMountingInstance;
+    } else {
+      return false;
+    }
   },
 
   /**
diff --git a/src/core/__tests__/ReactComponentLifeCycle-test.js b/src/core/__tests__/ReactComponentLifeCycle-test.js
index 736f2a02d7..7d61482da9 100644
--- a/src/core/__tests__/ReactComponentLifeCycle-test.js
+++ b/src/core/__tests__/ReactComponentLifeCycle-test.js
@@ -287,6 +287,23 @@ describe('ReactComponentLifeCycle', function() {
     );
   });
 
+  it('isMounted should return false when unmounted', function () {
+    var Component = React.createClass({
+      render: function() {
+        return <div/>;
+      }
+    });
+
+    var container = document.createElement('div');
+    var instance = React.render(<Component />, container);
+
+    expect(instance.isMounted()).toBe(true);
+
+    React.unmountComponentAtNode(container);
+
+    expect(instance.isMounted()).toBe(false);
+  });
+
   it('warns if findDOMNode is used inside render', function() {
     spyOn(console, 'error');
     var Component = React.createClass({