Получение ошибки при попытке выполнить проверку формы

Я создаю страницу регистрации для веб-сайта, но когда я захожу на страницу, я получаю сообщение об ошибке: Ошибка при выполнении процессора 'org.thymeleaf.spring5.processor.SpringInputGeneralFieldTagProcessor'

Контроллер

@RequestMapping(value = { "/signup" }, method = RequestMethod.POST)
    public ModelAndView createUser(@Valid AppUser appUser, BindingResult bindingResult) {
        ModelAndView model = new ModelAndView();
        AppUser appUserExists = appUserService.findByEmail(appUser.getEmail());

        if (appUserExists != null) {
            bindingResult.rejectValue("email", "error.user", "This email already exists!");
        }

        if (bindingResult.hasErrors()) {
            model.setViewName("user/signup");
        } else {
            appUserService.saveUser(appUser);
            model.addObject("msg", "User has been registered succesfully!");
            model.addObject("appuser", new AppUser());
            model.setViewName("user/login");
        }

        return model;
    }

Форма

<form class="form-horizontal" role="form" th:action="@{/signup}" th:object="${appUser}" method="post" style="border: 1px solid #ccc">
    <label for="name">First Name</label> 
    <input type="text" th:field="*{firstname}" class="form-control" id="firstname" placeholder="First Name" required autofocus /> 
    <label for="name">Last Name</label> 
    <input type="text" th:field="*{lastname}" class="form-control" id="lastname" placeholder="Last name" required autofocus /> 
    <label for="email"><b>Email</b></label>
    <input type="email" th:field="*{email}" class="form-control" id="email" placeholder="[email protected]" required autofocus /> 
    <label for="psw"><b>Password</b></label> 
    <input type="password" th:field="*{password}" class="form-control" id="password" placeholder="Password" required />
</form>

Класс пользователя приложения

@Entity
@Table(name = "user")
public class AppUser {

    @Id
    @Column(name = "id_user", length = 10, nullable = false, unique = true)
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Integer id;

    @Column(name = "firstname", length = 30, nullable = false, unique = true)
    private String firstName;

    @Column(name = "lastname", length = 30, nullable = false, unique = true)
    private String lastName;

    @Column(name = "email", length = 30, nullable = false, unique = true)
    private String email;

    @Column(name = "password", length = 500, nullable = false, unique = false)
    private String password;

    @Column(name = "active", nullable = false)
    private int active;

    public AppUser() {
        super();
    }

    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

       ....

}

Из того, что я вижу, здесь нет ничего плохого, поэтому я застрял. Любые идеи?