ImageType is not working

I have defined a field as ImageType this way (as documentation says):

->add('imagefile', 'formextra_image', array(
'base_path' => DIR.'/../../../../web/uploads/user_images/',
'base_uri' => '/uploads/user_images/',
'no_image_placeholder_uri' => '/img/noimage.png'

And it displays this error:

Fatal error: Call to a member function getPath() on a non-object in /Applications/MAMP/htdocs/myproject/vendor/bundles/SimpleThings/FormExtraBundle/Form/Type/ImageType.php on line 57

I have echoed the $data variable (the one throwing the error) and it contains just the value of the field, it's a string variable, but in the code is treated as an object.