Error when I try to export from Blender to Unreal

When I try using the Export Unreal Engine (.psk/.psa) addon built in to blender, I get an error message that says “Ambiguous root for UDK. More than one root bone is using deform.” How do I fix this?
P.S. I’m using blender 2.64

Edit: here’s the file http://www.pasteall.org/blend/18939

Attach or post a link to your blend file

Sorry about that, I just added a link to it.