加载模型时发生CodeIgniter HTTP 500错误

我在mac上使用MAMP本地托管一个CodeIgniter站点。
每当我尝试加载模型时，网站都会运行HTTP500错误，我不确定发生了什么。我怀疑我忽略了一些很小的东西，但我就是没有看到。

控制器：main.php

 <?php if ( ! defined('BASEPATH')) exit('No direct script access allowed');
 class Main extends CI_Controller {
public function index()
{
    //load content model
    $this->load->model('content_model');
    //grab content from the database
    //$data['content'] = $this->content_model->get_content(0);
    //display content
    $this->load->view('main_view');
}
 }
 ?>

型号：content_Model.php

 <?php 
 class Content_model extends CI_Model {
function __construct()
{
    //Call the Model constructor
    parent::__construct();
}
public function get_content($navID)
{
    $this->db->limit(1);
    $this->db->where('navID', $navID);
    $query = $this->db->get('pmlSiteContent');
    return $query->result();
}
 ?>

自动加载.php

 <?php  if ( ! defined('BASEPATH')) exit('No direct script access allowed');
 $autoload['packages'] = array();
 $autoload['libraries'] = array('database', 'session');
 $autoload['helper'] = array('url', 'form', 'file');
 $autoload['config'] = array();
 $autoload['language'] = array();
 $autoload['model'] = array();
 ?>

database.php

 <?php  if ( ! defined('BASEPATH')) exit('No direct script access allowed');
 $active_group = 'default';
 $active_record = TRUE;
 $db['default']['hostname'] = 'localhost:8889';
 $db['default']['username'] = 'root';
 $db['default']['password'] = 'root';
 $db['default']['database'] = 'pmlcms';
 $db['default']['dbdriver'] = 'mysql';
 $db['default']['dbprefix'] = '';
 $db['default']['pconnect'] = TRUE;
 $db['default']['db_debug'] = TRUE;
 $db['default']['cache_on'] = FALSE;
 $db['default']['cachedir'] = '';
 $db['default']['char_set'] = 'utf8';
 $db['default']['dbcollat'] = 'utf8_general_ci';
 $db['default']['swap_pre'] = '';
 $db['default']['autoinit'] = TRUE;
 $db['default']['stricton'] = FALSE;
 ?>

我错过了什么？非常感谢您的帮助，谢谢！