Why does data_class leads to LogicException?

Question

I have a form type (field_type) which extends text and have a data_class. Passing an entity instance to the form via event listener leads to a LogicException:

The form's view data is expected to be an instance of class Entity, but is a(n) string. You can avoid this error by setting the "data_class" option to null or by adding a view transformer that transforms a(n) string to an instance of Entity.

public function buildForm(FormBuilderInterface $builder, array $options)
{
    $builder->add('field', 'field_type', $opts);

    $builder->addEventListener(FormEvents::POST_SET_DATA, function (FormEvent $event) use($options)
    {
        ...
        $form = $event->getForm();
        $form->get('field')->setData($entity);
    });
}

$entity is an instance of the data_class. The form type has a view data transformer, too.

Field type:

public function buildForm(FormBuilderInterface $builder, array $options)
{
    $builder->addViewTransformer($this->viewTransformer, true);
}

public function setDefaultOptions(OptionsResolverInterface $resolver)
{
    $resolver->setDefaults(array(
        'data_class' => 'Entity',
        'invalid_message' => 'The given id is invalid!',
        'required' => true
    ));
}

public function getParent()
{
    return 'text';
}

Everything works fine except the part of the data_class. If I remove the data_class it works.

Why do I need to remove this part?