Symfony - 在表单的EventSubscriber中注入实体管理器

This is the first time I am working with EventListener of a form so I am struggling on how to inject EntityManager in it.

I have this formType called UserType and in this class I have an EventSubscriber AddDepartmentDegreeCourseFieldSubscriber which needs access to EntityManager

class UserType extends AbstractType
{
    /**
     * @param FormBuilderInterface $builder
     * @param array $options
     */
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $builder->addEventSubscriber(new AddProfileFieldSubscriber());
        $builder->addEventSubscriber(new AddDepartmentDegreeCourseFieldSubscriber());
    }

    /**
     * @param OptionsResolver $resolver
     */
    public function configureOptions(OptionsResolver $resolver)
    {
        $resolver->setDefaults(array(
            'data_class' => 'AppBundle\Entity\User'
        ));
    }
}

This is my services.yml

app.department_course_degree_subscriber:
    class: AppBundle\Form\EventListener\AddDepartmentDegreeCourseFieldSubscriber
    arguments: ["@doctrine.orm.entity_manager"]
    tags:
        - { name: kernel.event_subscriber }

The error I get is as following

Catchable Fatal Error: Argument 1 passed to AppBundle\Form\EventListener\AddDepartmentDegreeCourseFieldSubscriber::__construct() must be an instance of Doctrine\ORM\EntityManager, none given, called in /Users/shairyar/Sites/oxford-portal/src/AppBundle/Form/UserType.php on line 21 and defined

I know what the error means but I thought the service i registered in services.yml should inject the EntityManager so why I am getting this error? What am i missing here? Any help will be really appreciated.