User이 계속 null으로 들어와서 오류가 나네요

강의내용그대로 입력하였는데 왜그럴까요 ㅠㅠ

서비스부분

public void loan(BookLoanReq req) {
    Book book = bookRepstory.findByName(req.getBookName()).orElseThrow(IllegalArgumentException::new);
    if (userLoanHistoryRep.existsByBookNameAndIsReturn(book.getName(), false)){
        throw new IllegalArgumentException("대출중인 책입니다.");
    }
    User user=userRepstrory.findByName(req.getUserName()).orElseThrow(IllegalArgumentException::new);
        userLoanHistoryRep.save( new UserLoanHistory(user.getId(),book.getName(),false));
  }

public interface UserRepstrory extends JpaRepository<User,Long> {

    Optional<User> findByName(String name);
}

package com.group.libraryapp.domain.user;

import javax.persistence.*;

@Entity
public class User {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    public User(String name,Integer age) {
        if(name==null||name.isBlank()){
            throw new IllegalArgumentException(String.format("널값"));
        }
        this.name = name;
        this.age = age;
    }
    @Column(name="age",length = 20)
    private Integer age;
    @Column(name="name",nullable = false)
    private  String name;

    protected User(){
    }

    public Long getId() {
        return id;
    }

    public Integer getAge() {
        return age;
    }

    public String getName() {
        return name;
    }
    public void update(String name){
        this.name=name;
    }

    public User(Long id, Integer age, String name) {
        this.id = id;
        this.age = age;
        this.name = name;
    }
}

마지막으로 오류코드올려드릴게요

ava.lang.IllegalArgumentException: null

at java.base/java.util.Optional.orElseThrow(Optional.java:403) ~[na:na]

at com.group.libraryapp.service.book.BookService.loan(BookService.java:39) ~[main/:na]

at com.group.libraryapp.controller.book.BookController.loan(BookController.java:29) ~[main/:na]

at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:na]

at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:77) ~[na:na]

at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:na]

at java.base/java.lang.reflect.Method.invoke(Method.java:568) ~[na:na]