SonataAdminBundle: Autocomplete error with the basic setup (ManyToOne): Notice: Undefined index: read_only

I just setup the basic configuration like show in the documentation in order to make one form field autocomoplete (ManyToOne relationship) but I still get the following error: Notice: Undefined index: read_only.

/** * @param FormMapper $formMapper */ protected function configureFormFields(FormMapper $formMapper) { $formMapper ->add('figureValue') ->add('wordValue') ->add('pollingStation', 'sonata_type_model_autocomplete', array('property' => 'name')) ->add('prParty') ; }

/** * @ORM\ManyToOne(targetEntity="myEntity", inversedBy="prVoteCasts") * @ORM\JoinColumn(nullable=false) */ private $pollingStation;