i am making a simple javascript login form for wordpress. i have the form submitting to the following bit of php to handle the login:

<?php
get_header(); 
global $user_ID;


if (!$user_ID) {

    if($_POST){
        //We shall SQL escape all inputs
        $username = $wpdb->escape($_REQUEST['username']);
        $password = $wpdb->escape($_REQUEST['password']);
        $remember = $wpdb->escape($_REQUEST['rememberme']);

        if($remember) $remember = "true";
        else $remember = "false";
        $login_data = array();
        $login_data['user_login'] = $username;
        $login_data['user_password'] = $password;
        $login_data['remember'] = $remember;
        $user_verify = wp_signon( $login_data, false ); 
        //wp_signon is a wordpress function which authenticates a user. It accepts user info parameters as an array.

        if ( is_wp_error($user_verify) ) 
        {
           echo "<span class='error'>Invalid username or password. Please try again!</span>";
           exit();
         } else 
         {  
            echo "<script type='text/javascript'>window.location='". get_bloginfo('url') ."'</script>";
            exit();
          }
    } else { 

//get_header();

?>

any ideas on why i am getting the error? Cheers, Dan