Symfony2 save checkbox multiple

I try to save a multiple checkbox into my db.

this my code:

...
->add('typz', 'choice', array(
            'choices' => array('val1' => 'val1', 'val2' => 'val2', 'val3' => 'val3', 'val4' => 'val4'),
            'empty_value' => false,
            'expanded' => true,
            'required' => false,
            'multiple'  => true
        ))
...

In the entity i already tried field type simple_array, array, string. but i get always the error "The value you selected is not a valid choice".

Any help. thanks