Невозможно исправить: 'java.lang.String' на требуемый тип 'java.util.Collection'

org.springframework.validation.BindException: org.springframework.validation.BeanPropertyBindingResult: 1 errors
Field error in object 'content' on field 'stateCollection': rejected value [com.myapp.cmt.model.State[ id=3 ]]; codes [typeMismatch.content.stateCollection,typeMismatch.stateCollection,typeMismatch.java.util.Collection,typeMismatch]; arguments [org.springframework.context.support.DefaultMessageSourceResolvable: codes [content.stateCollection,stateCollection]; arguments []; default message [stateCollection]]; default message [Failed to convert property value of type 'java.lang.String' to required type 'java.util.Collection' for property 'stateCollection'; nested exception is java.lang.IllegalStateException:
Cannot convert value of type [java.lang.String] to required type [com.myapp.cmt.model.State] for property 'stateCollection[0]': no matching editors or conversion strategy found]

<strong>State</strong><br/>
<form:checkboxes path="stateCollection" items="${states}" itemLabel="name"/>

public class Content implements Serializable {
.......

    @JoinTable(name = "content_to_state", joinColumns = {
        @JoinColumn(name = "content_id", referencedColumnName = "id")}, inverseJoinColumns = {
        @JoinColumn(name = "state_id", referencedColumnName = "id")})
    @ManyToMany
    private Collection<State> stateCollection;

.....

    @XmlTransient
    public Collection<State> getStateCollection() {
        return stateCollection;
    }

    public void setStateCollection(Collection<State> stateCollection) {
        this.stateCollection = stateCollection;
    }

.....

...
@RequestMapping(value = "/{guid}/save", method = RequestMethod.POST)
public ModelAndView saveContent(@ModelAttribute("content") Content content, @PathVariable("guid") String guid) {
    try {
        // Save the modified object
        contentService.save(content);
    } catch (IllegalOrphanException ex) {

...

...
@Transactional
public void save(Content content) throws IllegalOrphanException, NonexistentEntityException, RollbackFailureException, Exception {
    try {
        utx.begin();
        em.merge(content);

        utx.commit();
    } catch (Exception ex) {

    } finally {
        if (em != null) {
            em.close();
        }
    }
}

...