// E-mail (generates "Value is required and can't be empty" - but what if the field should be optional? Where is this validator added?
        $email = new ElementEmail('email'); // If I change to "ElementText", there are no problems
        $email->setLabel('E-mail address');

        $email->setLabelAttributes(array(
                                        'for' => $email->getName(),
                                        'class' => 'some class'
                                   ));

        $email->setAttributes(array(
                                   'id' => $email->getName(),
                                   'placeholder' => 'E.g. andy@gmail.com',
                                   'class' => 'some class'
                              ));

// Gender dropdown (generates "Value is required and can't be empty error")
        $gender = new ElementSelect('gender');
        $gender->setLabel('Gender');
        $gender->setEmptyOption('Choose your gender'); // Remove this and the error disappears

        $gender->setLabelAttributes(array(
                                    'for' => $gender->getName(),
                                    'class' => 'some class'
                               ));

        $gender->setAttributes(array(
                                    'id' => $gender->getName(),
                                    'class' => 'some class'
                               ));

        $gender->setValueOptions(array(
                                      '1' => 'Male',
                                      '0' => 'Female' // This is selected on postback when the empty option is selected. If I change the value to 3, for instance, it works fine
                                 ));

$this->add($email);
        $this->add($gender);

public function registerAction() {
        $registerForm = new RegisterForm();
        $request = $this->getRequest();

        if ($request->isPost()) {
            $registerForm->setData($request->getPost());

            if ($registerForm->isValid()) {
                // Nothing here yet
            }

        }

        return new ViewModel(array('form' => $registerForm));
    }

$form = $this->form;
$email = $form->get('email');
$gender = $form->get('gender');

// E-mail
echo $this->formLabel()->openTag($email) . $email->getLabel() . $this->formLabel()->closeTag();
echo $this->formInput($email);
echo $this->formElementErrors($email);

// Gender
echo $this->formLabel()->openTag($gender) . $gender->getLabel() . $this->formLabel()->closeTag();
echo $this->formSelect($gender);
echo $this->formElementErrors($gender);

$form->prepare();  // this is another new thing and necessary

$input = $form->getInputFilter();
$e_filter = $input->get('email');
$e_filter->setRequired(false);

$g_filter = $input->get('gender');
$g_filter->setRequired(false);

$post = array('gender' => '');
$bool = in_array(0, $post);        // this is true
$bool = in_array(0, $post, true);  // but this is false

public function __construct($name = 'user') {
   ...
   $this->getInputFilter()->get('first_name')->setRequired(false);
}

->setAllowEmpty(false)

->setRequired(false)

@FormValidator({"name":"NotEmpty", "options":{"type":"string"}})