diff --git a/src/addons/__tests__/ReactFragment-test.js b/src/addons/__tests__/ReactFragment-test.js
index 237e10692d..f0f3297c75 100644
--- a/src/addons/__tests__/ReactFragment-test.js
+++ b/src/addons/__tests__/ReactFragment-test.js
@@ -60,6 +60,17 @@ describe('ReactFragment', function() {
     );
   });
 
+  it('should throw if a plain object looks like an old element', function() {
+    var oldEl = {_isReactElement: true, type: 'span', props: {}};
+    var container = document.createElement('div');
+    expect(() => ReactDOM.render(<div>{oldEl}</div>, container)).toThrow(
+      'Invariant Violation: Objects are not valid as a React child (found: ' +
+      'object with keys {_isReactElement, type, props}). It looks like ' +
+      'you\'re using an element created by a different version of React. ' +
+      'Make sure to use only one copy of React.'
+    );
+  });
+
   it('warns for numeric keys on objects as children', function() {
     spyOn(console, 'error');
 
diff --git a/src/shared/utils/traverseAllChildren.js b/src/shared/utils/traverseAllChildren.js
index 6733735867..488a092d28 100644
--- a/src/shared/utils/traverseAllChildren.js
+++ b/src/shared/utils/traverseAllChildren.js
@@ -181,19 +181,26 @@ function traverseAllChildrenImpl(
     } else if (type === 'object') {
       var addendum = '';
       if (__DEV__) {
+        addendum =
+          ' If you meant to render a collection of children, use an array ' +
+          'instead or wrap the object using createFragment(object) from the ' +
+          'React add-ons.';
+        if (children._isReactElement) {
+          addendum =
+            ' It looks like you\'re using an element created by a different ' +
+            'version of React. Make sure to use only one copy of React.';
+        }
         if (ReactCurrentOwner.current) {
           var name = ReactCurrentOwner.current.getName();
           if (name) {
-            addendum = ' Check the render method of `' + name + '`.';
+            addendum += ' Check the render method of `' + name + '`.';
           }
         }
       }
       var childrenString = String(children);
       invariant(
         false,
-        'Objects are not valid as a React child (found: %s). If you meant to ' +
-        'render a collection of children, use an array instead or wrap the ' +
-        'object using createFragment(object) from the React add-ons.%s',
+        'Objects are not valid as a React child (found: %s).%s',
         childrenString === '[object Object]' ?
           'object with keys {' + Object.keys(children).join(', ') + '}' :
           childrenString,