PHPUnit找不到Phalcon的Callback验证器（命名空间问题）

I'm using Phalcon 3.0.3 and PHP 7.0.15-0ubuntu0.16.04.4 with PHPUnit 6.0.8. If I try to use Phalcon\Validation\Validator\Callback I'll always end up with those kind of errros:

1) Test\ConfirmActivityTest::testConfirm Error: Class 'Phalcon\Validation\Validator\Callback' not found

So far the only way I found to solve this problem is to create a copy of that class in my codebase:

namespace Phapp\Validators;

/**
 * A workaround to fix the namespace problem of
 * Phalcon\Validation\Validator\Callback
 *
 */
class CallbackValidator extends \Phalcon\Validation\Validator
{

    /**
     * Executes the validation
     *
     * @param \Phalcon\Validation $validation
     * @param string $field
     * @return bool
     */
    public function validate(\Phalcon\Validation $validation, $field) {}

}

Is there any other workaround I could use to solve this problem? Is it a Phalcon bug?

Callback validator was added in 3.1.0. Docs were by mistake merged as i remember.