Argument #1 ($mailable) must be of type Illuminate\Contracts\Mail\Mailable

I try to send an email to a new user. But I get this error, but I don't understand what is wrong.

TypeError: Illuminate\Mail\PendingMail::send(): Argument #1 ($mailable) must be of type Illuminate\Contracts\Mail\Mailable, App\Events\UserStored given, called in /Users/kate_kimt/laravel/laravel_block/app/Listeners/SendMailNewUser.php on line 31 in file /Users/kate_kimt/laravel/laravel_block/vendor/laravel/framework/src/Illuminate/Mail/PendingMail.php on line 122

UserStored.php:

namespace App\Events;

use Illuminate\Broadcasting\Channel;
use Illuminate\Broadcasting\InteractsWithSockets;
use Illuminate\Broadcasting\PresenceChannel;
use Illuminate\Broadcasting\PrivateChannel;
use Illuminate\Contracts\Broadcasting\ShouldBroadcast;
use Illuminate\Foundation\Events\Dispatchable;
use Illuminate\Mail\Mailable;
use Illuminate\Queue\SerializesModels;
use App\Models\User;

class UserStored
{
    public User $user;
    use Dispatchable, InteractsWithSockets, SerializesModels;

    /**
     * Create a new event instance.
     *
     * @return void
     */
    public function __construct(User $user)
    {
        $this->user = $user;
    }

    /**
     * Get the channels the event should broadcast on.
     *
     * @return \Illuminate\Broadcasting\Channel|array
     */
    public function broadcastOn()
    {
        return new PrivateChannel('channel-name');
    }
}

SendMailNewUser.php:

namespace App\Listeners;

use App\Events\UserStored;
use Illuminate\Contracts\Queue\ShouldQueue;
use Illuminate\Queue\InteractsWithQueue;
use Illuminate\Support\Facades\Mail;
use Illuminate\Mail\Mailable;

class SendMailNewUser extends Mailable
{
    /**
     * Create the event listener.
     *
     * @return void
     */
    public function __construct()
    {
        //
    }

    /**
     * Handle the event.
     *
     * @param  \App\Events\UserStored  $event
     * @return void
     */
    public function handle(UserStored $event)
    {
        Mail::to($event->user->email)->send(new UserStored($event->user));
    }
}