Entities passed to the choice field must be managed

I create a new object and bind it to a form. The user fills out the form and goes to a preview page. I store the user responses in the session.

The problem crops up when I try to reload the object from the session when the user goes back to edit the form. I get :

Error: the Entities passed to the choice field must be managed.

Anyone have an idea of where i might be going wrong? Heres the code for the controllers.

public function previewdealAction(Request $request){

    $session = $this->getRequest()->getSession();
    $coupon = $session->get('coupon');
    $form = $this->createForm(new CouponType(), $coupon);

    if ($request->getMethod() == 'POST') {

        //bind the posted form values
        $form->bindRequest($request);

        //once a valid form is submitted ...
        if ($form->isValid()){
           //Proceed to Previewing deal
            $file = $coupon->getImage();
            $file->upload();
            $session->set('coupon', $coupon);

            $repository = $this->getDoctrine()
            ->getRepository('FrontendUserBundle:Coupon');
            $coupons = $repository->findAll();

            return $this->render('FrontendHomeBundle:Merchant:dealpreview.html.twig', array('coupon'=>$coupon, 'coupons'=>$coupons));

        }
    }

}
public function builddealAction(Request $request){

    $em = $this->get('doctrine')->getEntityManager();
    $user = $this->container->get('security.context')->getToken()->getUser();

    //check for a coupon session variable
    $session = $this->getRequest()->getSession();

    $coupon = $session->get('coupon');

    //If coupon is not set
    if($coupon == NULL){
        $coupon = new Coupon();
        $date = new \DateTime(date("Y-m-d H:i:s"));
        $coupon->setStartdate($date);
        $coupon->setPosterid($user);
        $session->set('coupon',