When my controller code below is executed, the routing shows __isInitialized__. I have no idea why this could happen.
Expected result : .....?city=1
Current result : ....?city[__isInitialized__]=1
Controller
if ($form->get('saveAndAdd')->isClicked()) {
    return $this->redirectToRoute('create_post', array('city' => 1));
}
Route
create_post:
    path: /create
    defaults:
        _controller: AcmeDemoBundle:Main:create