db_placeholders in drupal

/**
 * @file
 * Views handler: Node filter on "product-ness".
 */

/**
 * Filter nodes based on whether they have an entry in the uc_products table.
 */
class uc_product_handler_filter_product extends views_handler_filter_boolean_operator {
  function query() {
    $types = uc_product_types();   
    $this->query->add_field('node', 'type');
    $this->query->add_where($this->options['group'], "node.type " . (empty($this->value) ? "NOT " : "") . "IN (" . db_placeholders($types, 'varchar') . ")", $types);
  }
}

"where" isn't wroking as it should. I've tried to replace it with implode, but it looks like this function is also removing whitespace (so in SQL query it looks like "WHERE (...) node.typeINproduct"). I'm quite new to d7 database api and I would very gratefull if you share the solution

Changing

$this->query->add_where($this->options['group'], "node.type " . (empty($this->value) ? "NOT " : "") . "IN (" . db_placeholders($types, 'varchar') . ")", $types);

to

$this->query->add_where($this->options['group'], "node.type", $types);

solves the problem.