custom checkbox added with hook_form_alter always evaluates to false

hi there,

im trying to create a rule which has a condition for evaluating a checkbox. i've added the checkbox from my own module via hook_form_alter like this (a little simplified):

function foo_form_alter($form_id, &$form)
{
    $form_id['invite'] = array(
      '#tree' => true,
      'body' => array(
        '#type' => 'fieldset',
        '#collapsible' => false,
        '#title' => 'Invitation',
        '#tree' => true,
        'users' => array(
          '#type' => 'textfield',
          '#title' => t('Recpt.')
        ),
        'message' => array(
          '#type' => 'textarea',
          '#title' => t('Message'),
          '#value' => t('blub'),
        ),
        'send' => array(
          '#type' => 'checkbox',
          '#title' => t('Send invitation on next save'),
          '#value' => 0
        ),
        '#collapsed' => false
      )
    );

  return $form_id;
}

i've activated form events and created a rule on form submission with a condition which checks the value of the checkbox. i've used "invite[body][send]" as the form id and checked for value "true", "1" and "on", but all evaluate to false.