diff --git a/Source/core/events/MouseEvent.h b/Source/core/events/MouseEvent.h
index a61c73e..3783fa6 100644
--- a/Source/core/events/MouseEvent.h
+++ b/Source/core/events/MouseEvent.h
@@ -59,7 +59,7 @@
         int detail, int screenX, int screenY, int pageX, int pageY,
         int movementX, int movementY,
         bool ctrlKey, bool altKey, bool shiftKey, bool metaKey, unsigned short button,
-        PassRefPtr<EventTarget> relatedTarget, PassRefPtr<Clipboard>, bool isSimulated = false);
+        PassRefPtr<EventTarget> relatedTarget, PassRefPtrWillBeRawPtr<Clipboard>, bool isSimulated = false);
 
     static PassRefPtr<MouseEvent> create(const AtomicString& eventType, PassRefPtr<AbstractView>, const PlatformMouseEvent&, int detail, PassRefPtr<Node> relatedTarget);
 
@@ -91,12 +91,14 @@
     virtual bool isDragEvent() const OVERRIDE FINAL;
     virtual int which() const OVERRIDE FINAL;
 
+    virtual void trace(Visitor*) OVERRIDE;
+
 protected:
     MouseEvent(const AtomicString& type, bool canBubble, bool cancelable, PassRefPtr<AbstractView>,
         int detail, int screenX, int screenY, int pageX, int pageY,
         int movementX, int movementY,
         bool ctrlKey, bool altKey, bool shiftKey, bool metaKey, unsigned short button,
-        PassRefPtr<EventTarget> relatedTarget, PassRefPtr<Clipboard>, bool isSimulated);
+        PassRefPtr<EventTarget> relatedTarget, PassRefPtrWillBeRawPtr<Clipboard>, bool isSimulated);
 
     MouseEvent(const AtomicString& type, const MouseEventInit&);
 
@@ -106,7 +108,7 @@
     unsigned short m_button;
     bool m_buttonDown;
     RefPtr<EventTarget> m_relatedTarget;
-    RefPtr<Clipboard> m_clipboard;
+    RefPtrWillBeMember<Clipboard> m_clipboard;
 };
 
 class SimulatedMouseEvent FINAL : public MouseEvent {
@@ -114,6 +116,8 @@
     static PassRefPtr<SimulatedMouseEvent> create(const AtomicString& eventType, PassRefPtr<AbstractView>, PassRefPtr<Event> underlyingEvent);
     virtual ~SimulatedMouseEvent();
 
+    virtual void trace(Visitor*) OVERRIDE;
+
 private:
     SimulatedMouseEvent(const AtomicString& eventType, PassRefPtr<AbstractView>, PassRefPtr<Event> underlyingEvent);
 };