Configuring PE11 as external editor

I have Aperture 3.4.5 and Photoshop Elements 11.0 installed on my iMac (Mountain Lion). I have set the export preferences to launch PE11 as the external editor and set the file format to PSD 16 bit although I have tried other formats. When I go to Photos/Edit with Photoshop E;ements 11, Aperture launches PE 11 but the image never loads. I just have a blank workspace. Prior to upgrading to PE 11, I had PE9 and Aperture worked with that.

If I export an image from Aperture to the desktop then open it in PE11, that works OK so I think there is either a configuration problem or a bug bu I'm stuck. Any idea what to do next?