상품 수정부분에서 에러가 발생하는것 같습니다..

[질문 내용]
섹션7의 상품수정 부분을 수강중입니다!

강사님의 코드를 그대로 따라 쳤으나 상품 수정시 에러가 발생하는데 원인을 모르겠습니다.

다만, 아래와 같이 에러페이지가 나오나 홈으로 가서 다시 상품목록을 보면 수정이 반영되어었습니다...

package jpabook.jpashop.controller;

import jpabook.jpashop.domain.item.Book;
import jpabook.jpashop.domain.item.Item;
import jpabook.jpashop.service.ItemService;
import lombok.RequiredArgsConstructor;
import org.springframework.stereotype.Controller;
import org.springframework.ui.Model;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.ModelAttribute;
import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.bind.annotation.PostMapping;

import java.util.List;

@Controller
@RequiredArgsConstructor
public class ItemController {

    private final ItemService itemService;

    @GetMapping("/items/new")
    public String createForm(Model model) {
        model.addAttribute("form", new BookForm());
        return "items/createItemForm";
    }

    @PostMapping("/items/new")
    public String create(BookForm form) {

        Book book = new Book();
        book.setName(form.getName());
        book.setPrice(form.getPrice());
        book.setStockQuantity(form.getStockQuantity());
        book.setAuthor(form.getAuthor());
        book.setIsbn(form.getIsbn());

        itemService.saveItem(book);
        return "redirect:/";

    }

    @GetMapping("/items")
    public String list(Model model) {
        List<Item> items = itemService.findItem();
        model.addAttribute("items", items);
        return "items/itemList";
    }
    
    @GetMapping("items/{itemId}/edit")
    public String updateItemForm(@PathVariable("itemId") Long itemId, Model model) {
        Book item = (Book) itemService.findOne(itemId);

        BookForm form = new BookForm();
        form.setId(item.getId());
        form.setName(item.getName());
        form.setPrice(item.getPrice());
        form.setStockQuantity(item.getStockQuantity());
        form.setAuthor(item.getAuthor());
        form.setIsbn(item.getIsbn());

        model.addAttribute("form", form);
        return "items/updateItemForm";
    }

    @PostMapping("items/{itemId}/edit")
    public String updateItem(@PathVariable String itemId, @ModelAttribute("form") BookForm form) {

        Book book = new Book();
        book.setId(form.getId());
        book.setName(form.getName());
        book.setPrice(form.getPrice());
        book.setStockQuantity(form.getStockQuantity());
        book.setAuthor(form.getAuthor());
        book.setIsbn(form.getIsbn());

        itemService.saveItem(book);
        return "redirect:items";
    }

}

package jpabook.jpashop.controller;


import lombok.Getter;
import lombok.Setter;

@Getter @Setter
public class BookForm {

    private Long id;
    private String name;
    private int price;
    private int stockQuantity;
    private String author;
    private String isbn;
}