--- a/dom/base/nsDOMClassInfo.cpp
+++ b/dom/base/nsDOMClassInfo.cpp
@@ -9750,16 +9750,24 @@ nsHTMLPluginObjElementSH::GetPluginJSObj
                                             JSObject **plugin_obj,
                                             JSObject **plugin_proto)
 {
   *plugin_obj = nsnull;
   *plugin_proto = nsnull;
 
   JSAutoRequest ar(cx);
 
+  // NB: We need an AutoEnterCompartment because we can be called from
+  // nsObjectFrame when the plugin loads after the JS object for our content
+  // node has been created.
+  JSAutoEnterCompartment ac;
+  if (!ac.enter(cx, obj)) {
+    return NS_ERROR_UNEXPECTED;
+  }
+
   if (plugin_inst) {
     plugin_inst->GetJSObject(cx, plugin_obj);
     if (*plugin_obj) {
       *plugin_proto = ::JS_GetPrototype(cx, *plugin_obj);
     }
   }
 
   return NS_OK;