invoiceninja/app/Models/LookupUser.php

<?php

namespace App\Models;

use Eloquent;
use App\Models\User;

/**
 * Class ExpenseCategory.
 */
class LookupUser extends LookupModel
{
    /**
     * @var array
     */
    protected $fillable = [
        'lookup_account_id',
        'email',
        'user_id',
    ];

}