Laravel/Ardent - on save(), error: Relationship method must return an object of type Illuminate

I'm having some trouble getting my Laravel relationships to work out. In my application, there is a one-to-many relationship between users and ideas. (A user may have multiple ideas.) I'm using Ardent.

Here's my User model:

use Illuminate\Auth\UserTrait;
use Illuminate\Auth\UserInterface;
use Illuminate\Auth\Reminders\RemindableTrait;
use Illuminate\Auth\Reminders\RemindableInterface;

use LaravelBook\Ardent\Ardent;

class User extends Ardent implements UserInterface, RemindableInterface {

    use UserTrait, RemindableTrait;

    /**
     * The database table used by the model.
     *
     * @var string
     */
    protected $table = 'users';

    /**
     * The attributes excluded from the model's JSON form.
     *
     * @var array
     */
    protected $hidden = array('password', 'remember_token');
    protected $fillable = array('first_name', 'last_name', 'email', 'password');

    public $validation_errors;
    public $autoPurgeRedundantAttributes = true;
    public $autoHashPasswordAttributes = true;
    public $autoHydrateEntityFromInput = true;

    public static $passwordAttributes  = array('password');

    public static $rules = array(
        'first_name'            => 'required|between:1,16',
        'last_name'             => 'required|between:1,16',
        'email'                 => 'required|email|unique:users',
        'password'              => 'required|between:6,100'
    );

    public function ideas()
    {
        return $this->hasMany('Idea');
    }   
}

And here's my Idea model:

use LaravelBook\Ardent\Ardent;

class Idea extends Ardent {

    /**
     * The database table used by the model.
     *
     * @var string
     */
    protected $table = 'ideas';

    protected $fillable = array('title');

    public $validation_errors;
    public $autoPurgeRedundantAttributes = true;
    public $autoHydrateEntityFromInput = true;

    public static $rules = array(
        'title'            => 'required'
    );

    public function user()
    {
        return $this->belongsTo('User');
    }
}

Finally, here's my controller code:

class IdeasController extends BaseController {

    public function postInsert()
    {
        $idea = new Idea;

        $idea->user()->associate(Auth::user());

        if($idea->save())
        {
            return Response::json(array(
                'success' => true,
                'idea_id' => $idea->id,
                'title' => $idea->title),
                200
            );
        }
        else
        {
            return Response::json(array(
                'success' => false,
                'errors' => json_encode($idea->errors)),
                400
            );
        }
    }

}

$idea->save() throws the error:

{
  "error": {
    "type": "LogicException",
    "message": "Relationship method must return an object of type Illuminate\\Database\\Eloquent\\Relations\\Relation",
    "file": "\/var\/www\/3os\/vendor\/laravel\/framework\/src\/Illuminate\/Database\/Eloquent\/Model.php",
    "line": 2498
  }
}

At first, I was trying to set the user_id in the Idea like so:

$idea->user_id = Auth::id();

I then changed it to:

$idea->user()->associate(Auth::user());

But the results were the same.

Any suggestions would be much appreciated.