diff --git a/src/browser/ui/ReactDOMComponent.js b/src/browser/ui/ReactDOMComponent.js
index 9ce81b1468..ac932dc90b 100644
--- a/src/browser/ui/ReactDOMComponent.js
+++ b/src/browser/ui/ReactDOMComponent.js
@@ -147,6 +147,8 @@ function validateDangerousTag(tag) {
 function ReactDOMComponent(tag) {
   validateDangerousTag(tag);
   this._tag = tag;
+  this._renderedChildren = null;
+  this._previousStyleCopy = null;
 }
 
 ReactDOMComponent.displayName = 'ReactDOMComponent';
diff --git a/src/browser/ui/ReactDOMTextComponent.js b/src/browser/ui/ReactDOMTextComponent.js
index 6e63d9dbac..00e18fb851 100644
--- a/src/browser/ui/ReactDOMTextComponent.js
+++ b/src/browser/ui/ReactDOMTextComponent.js
@@ -50,6 +50,10 @@ assign(ReactDOMTextComponent.prototype, {
     // TODO: This is really a ReactText (ReactNode), not a ReactElement
     this._currentElement = text;
     this._stringText = '' + text;
+
+    // Properties
+    this._rootNodeID = null;
+    this._mountIndex = 0;
   },
 
   /**
diff --git a/src/core/ReactComponent.js b/src/core/ReactComponent.js
index 3803f891f5..8822ccb753 100644
--- a/src/core/ReactComponent.js
+++ b/src/core/ReactComponent.js
@@ -118,6 +118,10 @@ var ReactComponent = {
       // We keep the old element and a reference to the pending element
       // to track updates.
       this._currentElement = element;
+
+      this._rootNodeID = null;
+      this._mountIndex = 0;
+      this._mountDepth = 0;
     },
 
     /**
diff --git a/src/core/ReactCompositeComponent.js b/src/core/ReactCompositeComponent.js
index bd076698be..1d4c25c9f1 100644
--- a/src/core/ReactCompositeComponent.js
+++ b/src/core/ReactCompositeComponent.js
@@ -117,8 +117,11 @@ var ReactCompositeComponentMixin = assign({},
 
     this._pendingElement = null;
     this._pendingState = null;
+    this._pendingForceUpdate = false;
     this._compositeLifeCycleState = null;
 
+    this._renderedComponent = null;
+
     // Children can be either an array or more than one argument
     ReactComponent.Mixin.construct.apply(this, arguments);
 
diff --git a/src/core/instantiateReactComponent.js b/src/core/instantiateReactComponent.js
index ef5887974d..593f54c432 100644
--- a/src/core/instantiateReactComponent.js
+++ b/src/core/instantiateReactComponent.js
@@ -118,6 +118,14 @@ function instantiateReactComponent(node, parentCompositeType) {
   // Sets up the instance. This can probably just move into the constructor now.
   instance.construct(node);
 
+  // Internal instances should fully constructed at this point, so they should
+  // not get any new fields added to them at this point.
+  if (__DEV__) {
+    if (Object.preventExtensions) {
+      Object.preventExtensions(instance);
+    }
+  }
+
   return instance;
 }