CodeIgniter无法访问错误消息错误[关闭]

I've created this registration form for registering new users to a website using CodeIgniter. My problem is, whenever I enter a username that already exists in my database, instead of giving me my error message which explains this to the user, it instead gives me this error message:

Unable to access an error message corresponding to your field name

Here are snippets of the code from my controller. Any assistance will be appreciated:

function register()
{  
    $this->load->library('form_validation');  
    $this->form_validation->set_rules(
        'username', 
        'Username',
        'trim|required|alpha_numeric|min_length[6]|xss_clean'.
        '|strtolower|callback_username_not_exists'
    );  

    // function body here  
}

function username_not_exists($username)  
{  
    $this->form_validation->set_message('username','That %s already exists.');  

    if($this->User_model->check_exists_username($username))  
    {  
        return false;  
    }  
    else  
    {  
        return true;  
    }
}