Can not use localization on form validation rules

We have a set of rules like below:

PHP Code:

public $rules = array(
 'login'=>array(
 'email' => array(
 'field' => 'email',
 'label' => 'Email',
 'rules' => 'trim|valid_email|required'
 ), 


where we would like to replace the 'label' => 'Email' with $this->lang->line('user_email')

However it produces the following error:

Message: Constant expression contains invalid operations

Are we doing something wrong?