diff --git a/src/class/ReactClass.js b/src/class/ReactClass.js
index bae11f56219..b38bc4c72ce 100644
--- a/src/class/ReactClass.js
+++ b/src/class/ReactClass.js
@@ -22,7 +22,6 @@ var invariant = require('invariant');
 var keyMirror = require('keyMirror');
 var keyOf = require('keyOf');
 var monitorCodeUse = require('monitorCodeUse');
-var mapObject = require('mapObject');
 var warning = require('warning');
 
 var MIXINS_KEY = keyOf({mixins: null});
@@ -566,24 +565,26 @@ function mixStaticSpecIntoComponent(Constructor, statics) {
  * @param {object} two The second object
  * @return {object} one after it has been mutated to contain everything in two.
  */
-function mergeObjectsWithNoDuplicateKeys(one, two) {
+function mergeIntoWithNoDuplicateKeys(one, two) {
   invariant(
     one && two && typeof one === 'object' && typeof two === 'object',
-    'mergeObjectsWithNoDuplicateKeys(): Cannot merge non-objects'
+    'mergeIntoWithNoDuplicateKeys(): Cannot merge non-objects.'
   );
 
-  mapObject(two, function(value, key) {
-    invariant(
-      one[key] === undefined,
-      'mergeObjectsWithNoDuplicateKeys(): ' +
-      'Tried to merge two objects with the same key: `%s`. This conflict ' +
-      'may be due to a mixin; in particular, this may be caused by two ' +
-      'getInitialState() or getDefaultProps() methods returning objects ' +
-      'with clashing keys.',
-      key
-    );
-    one[key] = value;
-  });
+  for (var key in two) {
+    if (two.hasOwnProperty(key)) {
+      invariant(
+        one[key] === undefined,
+        'mergeIntoWithNoDuplicateKeys(): ' +
+        'Tried to merge two objects with the same key: `%s`. This conflict ' +
+        'may be due to a mixin; in particular, this may be caused by two ' +
+        'getInitialState() or getDefaultProps() methods returning objects ' +
+        'with clashing keys.',
+        key
+      );
+      one[key] = two[key];
+    }
+  }
   return one;
 }
 
@@ -604,7 +605,10 @@ function createMergedResultFunction(one, two) {
     } else if (b == null) {
       return a;
     }
-    return mergeObjectsWithNoDuplicateKeys(a, b);
+    var c = {};
+    mergeIntoWithNoDuplicateKeys(c, a);
+    mergeIntoWithNoDuplicateKeys(c, b);
+    return c;
   };
 }
 
diff --git a/src/core/__tests__/ReactCompositeComponent-test.js b/src/core/__tests__/ReactCompositeComponent-test.js
index 1510af60113..a8e41988cb9 100644
--- a/src/core/__tests__/ReactCompositeComponent-test.js
+++ b/src/core/__tests__/ReactCompositeComponent-test.js
@@ -586,7 +586,7 @@ describe('ReactCompositeComponent', function() {
     expect(function() {
       instance = ReactTestUtils.renderIntoDocument(instance);
     }).toThrow(
-      'Invariant Violation: mergeObjectsWithNoDuplicateKeys(): ' +
+      'Invariant Violation: mergeIntoWithNoDuplicateKeys(): ' +
       'Tried to merge two objects with the same key: `x`. This conflict ' +
       'may be due to a mixin; in particular, this may be caused by two ' +
       'getInitialState() or getDefaultProps() methods returning objects ' +
@@ -594,6 +594,26 @@ describe('ReactCompositeComponent', function() {
     );
   });
 
+  it('should not mutate objects returned by getInitialState()', function() {
+    var Mixin = {
+      getInitialState: function() {
+        return Object.freeze({mixin: true});
+      }
+    };
+    var Component = React.createClass({
+      mixins: [Mixin],
+      getInitialState: function() {
+        return Object.freeze({component: true});
+      },
+      render: function() {
+        return <span />;
+      }
+    });
+    expect(() => {
+      ReactTestUtils.renderIntoDocument(<Component />);
+    }).not.toThrow();
+  });
+
   it('should work with object getInitialState() return values', function() {
     var Component = React.createClass({
       getInitialState: function() {