Spring MVC form setting fields as null

I´m studying Spring MVC with Tiles 3, JPA and all other cool stuff. I created a form to send data, below is the form:

<form:form method="POST" action="/users/save" enctype="multipart/form-data" modelAttribute="formUsuario">
    <form:label path="username" for="username">Username</form:label>
    <form:input type="text" id="username" path="username" class="span4" value="${user.username}" />
    <form:label path="firstName" for="firstName">First name</form:label>
    <form:input type="text" id="firstName" path="firstName" class="span4" value="${user.firstName}" />
</form:form>

The method inside the controller to add a new record:

@Secured("ROLE_USER")
@RequestMapping(value = "/new", method = RequestMethod.GET)
public String getNew(@ModelAttribute("formUsuario") br.com.rodrigoferra.domain.User user) {
    logger.info("Add new user");
    return "users/edit";
}

The method to save the new record:

@Secured("ROLE_USER")
@RequestMapping(value = "/save", method = RequestMethod.POST)
public @ResponseBody String doSave(@ModelAttribute("formUsuario") br.com.rodrigoferra.domain.User user, BindingResult results, Model model) {
    if(results.hasErrors()) {
        return "users/edit";
    }
    user = userRepository.save(user);
    logger.info("Username: " + user.getUsername());
    return "redirect:/users/";
}

It is saving the record, but it is saving everything as Null! The data from the form is being received by the controller. I searched a lot and found the same examples as this, but I can't get mine to work!

Is there any configuration I've missed?

Thanks and sorry for my bad english.

Edited:

Users bean:

@Entity(name="users")
public class User implements Serializable {

    private static final long serialVersionUID = 1L;

    @Id
    @GeneratedValue( strategy = GenerationType.IDENTITY)
    private Long id;

    private String firstName;
    private String lastName;

    @Column(unique=true)
    private String username;

    private String password;
    @Transient
    private String passwordConfirmation;

    private Date birthdate;

    private String signature;
    private String email;
    private String sex;

    private int active;