--- a/testing/mochitest/runtestsb2g.py
+++ b/testing/mochitest/runtestsb2g.py
@@ -54,17 +54,19 @@ class B2GMochitest(Mochitest):
         if self.OOP:
             OOP_script = """
 let specialpowers = {};
 let loader = Cc["@mozilla.org/moz/jssubscript-loader;1"].getService(Ci.mozIJSSubScriptLoader);
 loader.loadSubScript("chrome://specialpowers/content/SpecialPowersObserver.js", specialpowers);
 let specialPowersObserver = new specialpowers.SpecialPowersObserver();
 specialPowersObserver.init();
 
-let mm = container.QueryInterface(Ci.nsIFrameLoaderOwner).frameLoader.messageManager;
+let fl = container.QueryInterface(Ci.nsIFrameLoaderOwner).frameLoader;
+fl.activateRemoteFrame();
+let mm = fl.messageManager;
 mm.addMessageListener("SPPrefService", specialPowersObserver);
 mm.addMessageListener("SPProcessCrashService", specialPowersObserver);
 mm.addMessageListener("SPPingService", specialPowersObserver);
 mm.addMessageListener("SpecialPowers.Quit", specialPowersObserver);
 mm.addMessageListener("SPPermissionManager", specialPowersObserver);
 
 mm.loadFrameScript(CHILD_LOGGER_SCRIPT, true);
 mm.loadFrameScript(CHILD_SCRIPT_API, true);