Embedded form generates array instead of ArrayCollection

I'm trying to achieve some reusable translation in my bundle. These are the relevant parts of my code:

TranslatorKeys.php:

...
/**
 * @var \ArrayCollection
 * 
 * @ORM\OneToMany(targetEntity="Jumi\TranslatorBundle\Entity\TranslatorStrings", mappedBy="key")
 */
private $strings;
/**
 * @param Doctrine\Common\Collections\ArrayCollection $strings
 */
public function setStrings(ArrayCollection $strings) { // THIS IS LINE 84
    $this->strings = $strings;
}
...

TranslatorStrings.php:

/**
 * @var integer
 * 
 * @ORM\ManyToOne(targetEntity="Jumi\TranslatorBundle\Entity\TranslatorLanguages", fetch="LAZY")
 * @ORM\JoinColumns({
 *      @ORM\JoinColumn(name="language_id", referencedColumnName="id", nullable=false)
 * })
 */
private $language;
/**
 * @var string
 *
 * @ORM\Column(name="_value", type="string", nullable=true)
 */
private $value;
/**
 * @param string $value
 */
public function setValue($value) {
    $this->value = $value;
}

EmbeddableTranslatorKeyType.php:

note: translator_strings type is defined in TranslatorStringsType.php

...
public function buildForm(FormBuilderInterface $builder, array $options) {
    $builder->add('strings', 'collection', array(
        'type' => 'translator_strings',
        'options' => array(
            'form_type' => $options['form_type'],
            'required' => $options['required']
        ),
        'prototype' => true,
        'allow_add' => true,
        'allow_delete' => true
    ));
}

public function setDefaultOptions(OptionsResolverInterface $resolver) {
    $resolver->setDefaults(array(
        'data_class' => 'Jumi\TranslatorBundle\Entity\TranslatorKeys',
        'form_type' => 'text'
    ));
    $resolver->addAllowedValues(array(
        'form_type' => array(
            'text',
            'textarea'
        )
    ));
}
...

TranslatorStringsType.php

...
public function buildForm(FormBuilderInterface $builder, array $options) {
    $builder->add('value', $options['form_type'], array(
        'label' => false,
        'required' => $options['required']
    ));
    $builder->add('language', 'hidden');
}

public function setDefaultOptions(OptionsResolverInterface $resolver) {
    $resolver->setDefaults(array(
        'form_type' => 'text',
        'data_class' => 'Jumi\TranslatorBundle\Entity\TranslatorStrings'
    ));
    $resolver->addAllowedValues(array(
        'form_type' => array(
            'text',
            'textarea'
        )
    ));
}
...

And i'm trying to use embeddable_translator_key this way:

note: title is a many to one relation on TranslatorKeys

$builder->add('title', 'embeddable_translator_key', array(
            'label' => 'label_title',
            'required' => false
        ));

Now the problem is, that i got this exception on binding the request to the form:

Catchable Fatal Error: Argument 1 passed to
Jumi\TranslatorBundle\Entity\TranslatorKeys::setStrings() must be an instance of
Doctrine\Common\Collections\ArrayCollection, array given, called in
E:\prog\SymfonyExperiment\trunk\Symfony\vendor\symfony\symfony\src\Symfony\Component\PropertyAccess\PropertyAccessor.php on line 350 and defined in
E:\prog\SymfonyExperiment\trunk\Symfony\src\Jumi\TranslatorBundle\Entity\TranslatorKeys.php line 84

It's an easy workaround if i just remove the parameter type definition of ArrayCollection on line 84 in the TranslatorKeys Entity, and convert $strings to ArrayCollection if it's an array, but i rather search for a proper sollution.

If i didn't provided enough information feel free to ask for more!

Symfony experts are welcome!