--- a/content/xbl/src/nsXBLProtoImpl.cpp
+++ b/content/xbl/src/nsXBLProtoImpl.cpp
@@ -153,20 +153,23 @@ nsXBLProtoImpl::InitTargetObjects(nsXBLP
 
   // Because our prototype implementation has a class, we need to build up a corresponding
   // class for the concrete implementation in the bound document.
   AutoJSContext cx;
   JS::Rooted<JSObject*> global(cx, sgo->GetGlobalJSObject());
   nsCOMPtr<nsIXPConnectJSObjectHolder> wrapper;
   JS::Rooted<JS::Value> v(cx);
 
-  // Make sure the interface object is created before the prototype object
-  // so that XULElement is hidden from content. See bug 909340.
-  bool defineOnGlobal = dom::XULElementBinding::ConstructorEnabled(cx, global);
-  dom::XULElementBinding::GetConstructorObject(cx, global, defineOnGlobal);
+  {
+    JSAutoCompartment ac(cx, global);
+    // Make sure the interface object is created before the prototype object
+    // so that XULElement is hidden from content. See bug 909340.
+    bool defineOnGlobal = dom::XULElementBinding::ConstructorEnabled(cx, global);
+    dom::XULElementBinding::GetConstructorObject(cx, global, defineOnGlobal);
+  }
 
   rv = nsContentUtils::WrapNative(cx, global, aBoundElement, v.address(),
                                   getter_AddRefs(wrapper));
   NS_ENSURE_SUCCESS(rv, rv);
 
   JS::Rooted<JSObject*> value(cx, &v.toObject());
 
   // All of the above code was just obtaining the bound element's script object and its immediate