diff --git a/dom/base/nsDOMClassInfo.cpp b/dom/base/nsDOMClassInfo.cpp
index 4ded20ab95a..9ee3222e547 100644
--- a/dom/base/nsDOMClassInfo.cpp
+++ b/dom/base/nsDOMClassInfo.cpp
@@ -4701,7 +4701,7 @@ nsDOMClassInfo::PostCreatePrototype(JSContext * cx, JSObject * proto)
   // XXX Is there a better way to check this?
   nsISupports *globalNative = XPConnect()->GetNativeOfWrapper(cx, global);
   nsCOMPtr<nsPIDOMWindow> piwin = do_QueryInterface(globalNative);
-  if(!piwin) {
+  if (!piwin) {
     return NS_OK;
   }
 
@@ -4709,6 +4709,15 @@ nsDOMClassInfo::PostCreatePrototype(JSContext * cx, JSObject * proto)
   if (win->IsClosedOrClosing()) {
     return NS_OK;
   }
+
+  // If the window is in a different compartment than the global object, then
+  // it's likely that global is a sandbox object whose prototype is a window.
+  // Don't do anything in this case.
+  if (win->FastGetGlobalJSObject() &&
+      global->compartment() != win->FastGetGlobalJSObject()->compartment()) {
+    return NS_OK;
+  }
+
   if (win->IsOuterWindow()) {
     // XXXjst: Do security checks here when we remove the security
     // checks on the inner window.