admin/content/webform page shows list of ALL content nodes, not webform only

I have only one node type assigned as containing webform: 'webform', as by default. But all the content nodes are shown on the webform management page. The problem is in the following code:

function webform_admin_content() {
  $webform_types = webform_variable_get('webform_node_types');
  $placeholders = implode(', ', array_fill(0, sizeof($webform_types), '%d'));
  $result = db_query(db_rewrite_sql("SELECT n.* FROM {node} n WHERE n.type IN ($placeholders)", $webform_types));

- which generates the following SQL statement: SELECT n.* FROM node n WHERE n.type IN (%d) - which for some weird reason returns all nodes.
I don't really understand what happenes here and have no time to investigate the problem, but the following change solves it:

  $webform_types = "'".implode(webform_variable_get('webform_node_types'), "', '")."'";
  $result = db_query(db_rewrite_sql("SELECT n.* FROM {node} n WHERE n.type IN ($webform_types)"));