Error : Source Model not found while creating a new attribute in customer entity

I wrote a simple sql upgrade script to add a custom attribute to the customer table.
My script :

< ?php
$installer = $this;

$installer->startSetup();

$installer->addAttribute('customer', 'whatsapp_active', array(
    'type'       => 'static',
    'input'      => 'boolean',
    'label'      => 'Is Customer Active',
    'default' => '1',
    'visible' => true,
    'required' => false,
    'user_defined' => true,
));

Mage::getSingleton('eav/config')
    ->getAttribute('customer', 'whatsapp_active')
    ->setData('used_in_forms', array('adminhtml_customer'))
    ->save();

$installer->endSetup();

I get the error :

a:5:{i:0;s:57:"Source model "" not found for attribute "whatsapp_status"";i:1;s:2983:"#0 /var/www/html/fresh/app/code/core/Mage/Eav/Model/Entity/Attribute/Abstract.php(387): Mage::exception('Mage_Eav', 'Source model ""...')
#1 /var/www/html/fresh/app/code/core/Mage/Adminhtml/Block/Widget/Form.php(201): Mage_Eav_Model_Entity_Attribute_Abstract->getSource()

Now, even when I have removed the file, then also I get the same error.

PS : I have changed, my core_resource table code customer_setup versions accordingly.

So, what is the thing which I am missing?