warning undefined max_number

The following code through undefined warning line number 1546

    if (isset($form['identity']['type']['#default_value'])) {
       $max_number = variable_get('og_max_groups_' . $form['identity']['type']['#default_value'],'');
    }

This should be like following.

  if (isset($form['identity']['type']['#default_value'])) {
    $max_number = variable_get('og_max_groups_' . $form['identity']['type']['#default_value'],'');
  }else{
    $max_number='';
  }