diff --git a/atom/browser/lib/guest-window-manager.coffee b/atom/browser/lib/guest-window-manager.coffee
index eea26462b68d..53bbb735b044 100644
--- a/atom/browser/lib/guest-window-manager.coffee
+++ b/atom/browser/lib/guest-window-manager.coffee
@@ -5,7 +5,7 @@ frameToGuest = {}
 
 # Copy attribute of |parent| to |child| if it is not defined in |child|.
 mergeOptions = (child, parent) ->
-  for own key, value of parent when key not in Object.keys child
+  for own key, value of parent when key not of child
     if typeof value is 'object'
       child[key] = mergeOptions {}, value
     else