diff --git a/atom/browser/lib/guest-window-manager.coffee b/atom/browser/lib/guest-window-manager.coffee
index c311e01cf48..9b4490c4e08 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 child
+  for own key, value of parent when key not in Object.keys child
     if typeof value is 'object'
       child[key] = mergeOptions {}, value
     else
diff --git a/spec/chromium-spec.coffee b/spec/chromium-spec.coffee
index 7aecde218da..211de346846 100644
--- a/spec/chromium-spec.coffee
+++ b/spec/chromium-spec.coffee
@@ -82,6 +82,15 @@ describe 'chromium feature', ->
       window.addEventListener 'message', listener
       b = window.open "file://#{fixtures}/pages/window-open-size.html", '', 'show=no'
 
+    it 'does not override child options', (done) ->
+      size = {width: 350, height: 450}
+      listener = (event) ->
+        assert.equal event.data, "size: #{size.width} #{size.height}"
+        b.close()
+        done()
+      window.addEventListener 'message', listener
+      b = window.open "file://#{fixtures}/pages/window-open-size.html", '', "show=no,width=#{size.width},height=#{size.height}"
+
   describe 'window.opener', ->
     @timeout 10000