JSF Error de validación: el valor no es válido [duplicado]

Esta pregunta ya tiene una respuesta aquí:

Sé que esto parece ser común, pero estoy perdido con eso. Se produce al hacer clic en el botón Agregar en Assessment.jsf. De todos modos, he adjuntado lo que creo que son las secciones relevantes.

FWIW, AssessmentType.equals () no se activa cuando depuro.

Gracias por adelantado

j_idt38:j_idt47:j_idt48: Validation Error: Value is not valid

Assessment.xhtml:

        <h:form>
            <h:selectOneMenu value="#{assessmentBean.assessmentField}">
                <f:selectItems value="#{assessmentBean.assessment.type.fields}" />
            </h:select,OneMenu>

            <h:commandButton value="Add" action="#{assessmentBean.doAddField}">
                <f:param name="assessmentId"
                    value="#{assessmentBean.assessment.id}" />
            </h:commandButton>
        </h:form>

Assessment.jsf:

<form id="j_idt38:j_idt47" name="j_idt38:j_idt47" method="post" action="/jsf-web/edit/assessment.jsf" enctype="application/x-www-form-urlencoded"> 
<input type="hidden" name="j_idt38:j_idt47" value="j_idt38:j_idt47" /> 
<select name="j_idt38:j_idt47:j_idt48" size="1">    <option value="1">Presenting Condition</option> 
    <option value="2">Problem Duration</option> 
</select> 
<script type="text/javascript" src="/jsf-web/javax.faces.resource/jsf.js.jsf?ln=javax.faces"></script>
<input type="submit" name="j_idt38:j_idt47:j_idt50" value="Add" onclick="mojarra.jsfcljs(document.getElementById('j_idt38:j_idt47'),{'j_idt38:j_idt47:j_idt50':'j_idt38:j_idt47:j_idt50','assessmentId':'1'},'');return false" /><input type="hidden" name="javax.faces.ViewState" id="javax.faces.ViewState" value="3431661972220941645:6952134510589038883" autocomplete="off" /> 
</form> 

AssessmentType.java:

import java.util.List;

import javax.persistence.CascadeType;
import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.OneToMany;
import javax.validation.constraints.NotNull;

import lombok.Data;

import org.hibernate.envers.Audited;

@Audited
@Data
@Entity
public class AssessmentType implements Comparable<AssessmentType> {

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

    @NotNull
    private String name;

    @OneToMany( fetch=FetchType.EAGER, cascade = {CascadeType.PERSIST, CascadeType.MERGE}, targetEntity=AssessmentField.class )
    private List<AssessmentField> fields;

    @Override
    public int compareTo(final AssessmentType o) {
        return getId().compareTo(o.getId());
    }

    @Override
    public String toString() {
        return getName();
    }
}

AssessmentFieldConverter.java

import javax.faces.component.UIComponent;
import javax.faces.context.FacesContext;
import javax.faces.convert.Converter;
import javax.faces.convert.FacesConverter;
import javax.naming.InitialContext;
import javax.naming.NamingException;

import com.htu.fizio.api.AssessmentFieldManager;
import com.htu.fizio.domain.AssessmentField;

@FacesConverter(forClass = AssessmentField.class)
public class AssessmentFieldConverter implements Converter {

    AssessmentFieldManager<AssessmentField> assessmentFieldManager;

    @SuppressWarnings({ "unchecked", "rawtypes" })
    @Override
    public Object getAsObject(FacesContext ctx, UIComponent component, String value) {
        try {
            final InitialContext ic = new InitialContext();

            assessmentFieldManager = (AssessmentFieldManager) ic.lookup("fizio/AssessmentFieldManagerImpl/local");

            return assessmentFieldManager.find(Long.valueOf(value));
        } catch (NamingException e) {
            e.printStackTrace();
        }

        return null;
    }

    @Override
    public String getAsString(FacesContext ctx, UIComponent component, Object value) {
        return String.valueOf(((AssessmentField) value).getId());
    }
}

AssessmentBean.java

    import java.util.List;

    import javax.annotation.PostConstruct;
    import javax.ejb.EJB;
    import javax.faces.bean.ManagedBean;
    import javax.faces.bean.ManagedProperty;
    import javax.faces.bean.SessionScoped;

    import lombok.Getter;
    import lombok.Setter;

    import com.htu.fizio.api.AssessmentManager;
    import com.htu.fizio.domain.Assessment;
    import com.htu.fizio.domain.AssessmentField;
    import com.htu.fizio.domain.AssessmentFieldValue;
    import com.htu.fizio.jsf.faces.FacesUtil;

...    

@PostConstruct
public void init() {
    if (FacesUtil.containsKey("assessmentId")) {
        final Long id = Long.parseLong(FacesUtil.get("assessmentId"));

        assessment = assessmentManager.find(id);
    } else {
        assessment = new Assessment();
    }
}

    public String doAddField() {
        final AssessmentFieldValue value = new AssessmentFieldValue();

        value.setField(assessmentField);
        value.setValue("");

        assessment.getFieldValues().add(value);

        assessmentManager.save(assessment);

        return "/edit/assessment";
    }

Editar

Acabo de notar esto al depurar, ¿es probable que sea sospechoso?:

Daemon Thread [HandshakeCompletedNotify-Thread] (Suspended (exception ConcurrentModificationException)) 
    HashMap$EntryIterator(HashMap$HashIterator<E>).nextEntry() line: 793    
    HashMap$EntryIterator.next() line: 834  
    HashMap$EntryIterator.next() line: 832  
    SSLSocketImpl$NotifyHandshakeThread.run() line: 2214