diff --git a/atom/browser/lib/guest-window-manager.coffee b/atom/browser/lib/guest-window-manager.coffee
index 3b331260b1..7ed42ecff2 100644
--- a/atom/browser/lib/guest-window-manager.coffee
+++ b/atom/browser/lib/guest-window-manager.coffee
@@ -32,9 +32,9 @@ createGuest = (embedder, url, frameName, options) ->
     guest.once 'closed', ->
       delete frameToGuest[frameName]
 
-  ipc.on 'ATOM_SHELL_GUEST_WINDOW_MANAGER_WINDOW_OPENER_POSTMESSAGE', (event, method, args...) ->
-    if embedder.getUrl().indexOf(args[1]) is 0 or args[1] is '*'
-      embedder.send 'ATOM_SHELL_GUEST_WINDOW_MANAGER_WINDOW_POSTMESSAGE', args[0], args[1]
+  ipc.on 'ATOM_SHELL_GUEST_WINDOW_MANAGER_WINDOW_OPENER_POSTMESSAGE', (event, message, targetOrigin) ->
+    if embedder.getUrl().indexOf(targetOrigin) is 0 or targetOrigin is '*'
+      embedder.send 'ATOM_SHELL_GUEST_WINDOW_MANAGER_WINDOW_POSTMESSAGE', message, targetOrigin
 
   guest.id
 
@@ -55,11 +55,11 @@ ipc.on 'ATOM_SHELL_GUEST_WINDOW_MANAGER_WINDOW_METHOD', (event, guestId, method,
   return unless BrowserWindow.windows.has guestId
   BrowserWindow.windows.get(guestId)[method] args...
 
-ipc.on 'ATOM_SHELL_GUEST_WINDOW_MANAGER_WINDOW_POSTMESSAGE', (event, guestId, method, args...) ->
+ipc.on 'ATOM_SHELL_GUEST_WINDOW_MANAGER_WINDOW_POSTMESSAGE', (event, guestId, message, targetOrigin) ->
   return unless BrowserWindow.windows.has guestId
   window = BrowserWindow.windows.get(guestId)
-  if window.getUrl().indexOf(args[1]) is 0 or args[1] is '*'
-    window.send 'ATOM_SHELL_GUEST_WINDOW_MANAGER_WINDOW_POSTMESSAGE', args[0], args[1]
+  if window.getUrl().indexOf(targetOrigin) is 0 or targetOrigin is '*'
+    window.send 'ATOM_SHELL_GUEST_WINDOW_MANAGER_WINDOW_POSTMESSAGE', message, targetOrigin
 
 ipc.on 'ATOM_SHELL_GUEST_WINDOW_MANAGER_WEB_CONTENTS_METHOD', (event, guestId, method, args...) ->
   return unless BrowserWindow.windows.has guestId
diff --git a/atom/renderer/lib/override.coffee b/atom/renderer/lib/override.coffee
index eba0e9c982..a727da59c4 100644
--- a/atom/renderer/lib/override.coffee
+++ b/atom/renderer/lib/override.coffee
@@ -5,11 +5,9 @@ remote = require 'remote'
 # Window object returned by "window.open".
 class FakeWindow
   constructor: (@guestId) ->
-    that = this
-
-    ipc.on 'ATOM_SHELL_GUEST_WINDOW_MANAGER_WINDOW_CLOSED', (guestId) ->
-      if guestId is that.guestId
-        that.closed = true
+    ipc.on 'ATOM_SHELL_GUEST_WINDOW_MANAGER_WINDOW_CLOSED', (guestId) =>
+      if guestId is @guestId
+        @closed = true
 
   close: ->
     ipc.send 'ATOM_SHELL_GUEST_WINDOW_MANAGER_WINDOW_CLOSE', @guestId
@@ -20,8 +18,8 @@ class FakeWindow
   blur: ->
     ipc.send 'ATOM_SHELL_GUEST_WINDOW_MANAGER_WINDOW_METHOD', @guestId, 'blur'
 
-  postMessage: (args...) ->
-    ipc.send 'ATOM_SHELL_GUEST_WINDOW_MANAGER_WINDOW_POSTMESSAGE', @guestId, 'postMessage', args[0], args[1]
+  postMessage: (message, targetOrigin) ->
+    ipc.send 'ATOM_SHELL_GUEST_WINDOW_MANAGER_WINDOW_POSTMESSAGE', @guestId, message, targetOrigin
 
   eval: (args...) ->
     ipc.send 'ATOM_SHELL_GUEST_WINDOW_MANAGER_WEB_CONTENTS_METHOD', @guestId, 'executeJavaScript', args...
@@ -73,8 +71,8 @@ window.prompt = ->
   throw new Error('prompt() is and will not be supported in atom-shell.')
 
 window.opener =
-  postMessage: (args...) ->
-    ipc.send 'ATOM_SHELL_GUEST_WINDOW_MANAGER_WINDOW_OPENER_POSTMESSAGE', 'postMessage', args[0], args[1]
+  postMessage: (message, targetOrigin) ->
+    ipc.send 'ATOM_SHELL_GUEST_WINDOW_MANAGER_WINDOW_OPENER_POSTMESSAGE', message, targetOrigin
 
-ipc.on 'ATOM_SHELL_GUEST_WINDOW_MANAGER_WINDOW_POSTMESSAGE', (data, origin) ->
-  window.postMessage(data, origin)
+ipc.on 'ATOM_SHELL_GUEST_WINDOW_MANAGER_WINDOW_POSTMESSAGE', (message, targetOrigin) ->
+  window.postMessage(message, targetOrigin)