diff --git a/browser/api/lib/dialog.coffee b/browser/api/lib/dialog.coffee
index fa606e1ddbdd..59568bcf5b4e 100644
--- a/browser/api/lib/dialog.coffee
+++ b/browser/api/lib/dialog.coffee
@@ -31,7 +31,7 @@ module.exports =
     binding.showSaveDialog window, options.title, options.defaultPath
 
   showMessageBox: (window, options) ->
-    if window?.constructor isnt BrowserWindow
+    if window? and window.constructor isnt BrowserWindow
       options = window
       window = null