PDO lastInsertID Fatal Error -

i have class named db. when insert in table, returning error message:

call member function lastinsertid() on null in /home/...

everything ok why problem occurs don't have idea. codes:

class db {    public static $pdo = null;    public static $last_stmt = null;  public static function instance() {     return ($pdo == null ? self::init() : $pdo); }  public static function init() {     $pdo = new pdo("mysql:host=" . mysql_host . ";dbname=" . mysql_db, mysql_user, mysql_pass);     $pdo->exec("set names utf8");     $pdo->setattribute(pdo::attr_default_fetch_mode, pdo::fetch_obj);     return $pdo; }  public static function query($query, $bindings = null) {     if( is_null($bindings) )      {         if( !($last_stmt = self::instance()->query($query)) )          {             return false;         }      }     else     {         $last_stmt = self::prepare($query);         if( !$last_stmt->execute($bindings) )          {             return false;         }      }      return $last_stmt; }  public static function insert($query, $bindings = null) {     if( !($stmt = self::query($query, $bindings)) )      {         return false;     }      return $pdo->lastinsertid(); } 

}