Hello I want to do a manytomany relation two entity and i want using h:selectOneMenu but its giving this error :

avax.faces.component.UIInput updateModel
SEVERE: /employee.xhtml @26,76 value="#{employeeHome.instance.projects}": java.lang.IllegalArgumentException: argument type mismatch
java.lang.IllegalArgumentException: argument type mismatch 

package org.domain.ManytoMany.entity;

import java.io.Serializable;
import java.util.Set;

import javax.persistence.Entity;
import javax.persistence.Id;
import javax.persistence.GeneratedValue;
import javax.persistence.ManyToMany;
import javax.persistence.Version;
import org.hibernate.validator.Length;

@Entity
public class Employee implements Serializable
{
    // seam-gen attributes (you should probably edit these)
    private Long id;
    private Integer version;
    private String name;
    protected Set <Project> projects;
    // add additional entity attributes

    // seam-gen attribute getters/setters with annotations (you probably should edit)

    @Id @GeneratedValue
    public Long getId() {
        return id;
    }

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

    @Version
    public Integer getVersion() {
        return version;
    }

    private void setVersion(Integer version) {
        this.version = version;
    }

    @Length(max = 20)
    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    @ManyToMany(mappedBy="employees")
     public Set<Project> getProjects() {
          return projects;
     }

     public void setProjects(Set<Project> projects) {
          this.projects = projects;
     }

}

package org.domain.ManytoMany.entity;

import java.io.Serializable;
import java.util.Set;

import javax.persistence.Entity;
import javax.persistence.Id;
import javax.persistence.GeneratedValue;
import javax.persistence.ManyToMany;
import javax.persistence.Version;
import org.hibernate.validator.Length;

@Entity
public class Project implements Serializable
{
    // seam-gen attributes (you should probably edit these)
    private Long id;
    private Integer version;
    private String name;
    protected Set <Employee> employees;
    // add additional entity attributes

    // seam-gen attribute getters/setters with annotations (you probably should edit)

    @Id @GeneratedValue
    public Long getId() {
        return id;
    }

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

    @Version
    public Integer getVersion() {
        return version;
    }

    private void setVersion(Integer version) {
        this.version = version;
    }

    @Length(max = 20)
    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    @ManyToMany
     public Set<Employee> getEmployees() {
          return employees;
     }

     public void setEmployees(Set<Employee> employees) {
          this.employees = employees;
     }

}

<!DOCTYPE composition PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
    "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<ui:composition xmlns="http://www.w3.org/1999/xhtml"
    xmlns:s="http://jboss.com/products/seam/taglib"
    xmlns:ui="http://java.sun.com/jsf/facelets"
    xmlns:f="http://java.sun.com/jsf/core"
    xmlns:h="http://java.sun.com/jsf/html"
    xmlns:rich="http://richfaces.org/rich"
    template="layout/template.xhtml">

<ui:define name="body">

    <h:form id="employeeForm">

        <rich:panel>
            <f:facet name="header">employee</f:facet>

            <s:decorate id="nameField" template="layout/edit.xhtml">
                <ui:define name="label">Name</ui:define>
                <h:inputText id="name" required="true"
                             value="#{employeeHome.instance.name}"/>
            </s:decorate>
            
            <s:decorate id="projectField" template="layout/edit.xhtml">
                <ui:define name="label">Project</ui:define>
                <h:selectOneMenu value="#{employeeHome.instance.projects}">
                   <s:selectItems var="project" value="#{projectList.resultList}" label="#{project.name}"></s:selectItems>
                  <s:convertEntity></s:convertEntity>
                </h:selectOneMenu>
            </s:decorate>

            <div style="clear:both"/>

        </rich:panel>

        <div class="actionButtons">
            <h:commandButton id="save"
                          value="Save"
                         action="#{employeeHome.persist}"
                       rendered="#{!employeeHome.managed}"/>
            <h:commandButton id="update"
                          value="Save"
                         action="#{employeeHome.update}"
                       rendered="#{employeeHome.managed}"/>
            <h:commandButton id="delete"
                          value="Delete"
                         action="#{employeeHome.remove}"
                      immediate="true"
                       rendered="#{employeeHome.managed}"/>
            <s:button propagation="end"
                               id="cancel"
                            value="Cancel"
                             view="/employeeList.xhtml"/>
        </div>

    </h:form>

</ui:define>

</ui:composition>

what is wrong ?