PHP Fatal error: I'm trying to implement a Form\AbstractType in my Symfony2 application

I get the following error:

Fatal error: Declaration of Bean\OauthServerBundle\Form\Type\AuthorizeFormType::buildForm() must be compatible with Symfony\Component\Form\FormTypeInterface::buildForm(Symfony\Component\Form\FormBuilderInterface $builder, array $options) in src/Bean/OauthServerBundle/Form/Type/AuthorizeFormType.php on line 25

Not sure why I'm getting this Error. AbstractType::buildForm() takes a FormBuilderInterface, and Symfony2 implements FormBuilderInterface for FormBuilder.

Here's the content of my Source:

<?php
namespace Bean\OauthServerBundle\Form\Type;

use Symfony\Component\Form\FormBuilder;
use Symfony\Component\Form\AbstractType;

class AuthorizeFormType extends AbstractType
{
    public function buildForm(FormBuilder $builder, array $options)
    {
        /* some code ... */
   }

   /* more code ... */
}