Laravel One to One relation error don't know why?

Error Showing

Symfony\Component\HttpFoundation\Response::setContent(): Argument #1 ($content) must be of type ?string, Illuminate\Database\Eloquent\Relations\HasOne given, called in G:\Laravel\practice-1\vendor\laravel\framework\src\Illuminate\Http\Response.php on line 72

User Model

<?php

namespace App\Models;

use Illuminate\Contracts\Auth\MustVerifyEmail;
use Illuminate\Database\Eloquent\Factories\HasFactory;
use Illuminate\Foundation\Auth\User as Authenticatable;
use Illuminate\Notifications\Notifiable;
use Laravel\Sanctum\HasApiTokens;

class User extends Authenticatable
{
    use HasApiTokens, HasFactory, Notifiable;

    /**
     * The attributes that are mass assignable.
     *
     * @var array<int, string>
     */
    protected $fillable = [
        'name',
        'email',
        'password',
    ];

    /**
     * The attributes that should be hidden for serialization.
     *
     * @var array<int, string>
     */
    protected $hidden = [
        'password',
        'remember_token',
    ];

    /**
     * The attributes that should be cast.
     *
     * @var array<string, string>
     */
    protected $casts = [
        'email_verified_at' => 'datetime',
    ];


    public function book()
    {
        # code...
        return $this->hasOne(Book::class);
    }
}

Book Migration

<?php

use Illuminate\Database\Migrations\Migration;
use Illuminate\Database\Schema\Blueprint;
use Illuminate\Support\Facades\Schema;

class CreateBooksTable extends Migration
{
    /**
     * Run the migrations.
     *
     * @return void
     */
    public function up()
    {
        Schema::create('books', function (Blueprint $table) {
            $table->id();
            $table->integer('user_id')->unsigned();
            $table->string('title');
            $table->string('abut');
            $table->float('amount', 8, 2);
            $table->timestamps();
        });
    }

    /**
     * Reverse the migrations.
     *
     * @return void
     */
    public function down()
    {
        Schema::dropIfExists('books');
    }
}

Route::get('/user/{id}/book', function ($id) {
    return User::find($id)->book();
});

I have no idea why I am seeing this error. I have also looked at the documentation but no help was found.