Getting hibernate error when using criteria

I have a class Item and Category

Class Item
{
     private long ItemId;
     private String ItemName;
     private String ItemDescription;
     private String ItemWebSite;
     private String Condition;
     private long Price;
     private String Brand;
     private String AttributeDescription;
     private Category ProductCategory;
     private Set <Picture> Pictures= new HashSet <Picture>();
     //getters and setters
}

Class Category
{
        private long CategoryId;
    private String CategoryName;
    private Category ParentCategory;
    private Set <Category> SubCategory=new HashSet <Category> ();
    private Set <Attribute> AllAttributes= new HashSet  <Attribute>();
         //getters and setters
}

But when I doing the follwing query:

long id=841;

Criteria crit =session.createCriteria(Item.class).add(Restrictions.eq("ProductCategory",id));

I am getting this error:

Caused by: org.hibernate.PropertyAccessException: IllegalArgumentException occurred calling getter of com.BiddingSystem.Models.Category.CategoryId
    at org.hibernate.property.BasicPropertyAccessor$BasicGetter.get(BasicPropertyAccessor.java:198)
    at org.hibernate.tuple.entity.AbstractEntityTuplizer.getIdentifier(AbstractEntityTuplizer.java:230)
    at org.hibernate.persister.entity.AbstractEntityPersister.getIdentifier(AbstractEntityPersister.java:3852)
    at org.hibernate.persister.entity.AbstractEntityPersister.isTransient(AbstractEntityPersister.java:3560)
    at org.hibernate.engine.ForeignKeys.isTransient(ForeignKeys.java:204)
    at org.hibernate.engine.ForeignKeys.getEntityIdentifierIfNotUnsaved(ForeignKeys.java:243)
    at org.hibernate.type.EntityType.getIdentifier(EntityType.java:449)
    at org.hibernate.type.ManyToOneType.nullSafeSet(ManyToOneType.java:141)
    at org.hibernate.loader.Loader.bindPositionalParameters(Loader.java:1769)
    at org.hibernate.loader.Loader.bindParameterValues(Loader.java:1740)
    at org.hibernate.loader.Loader.prepareQueryStatement(Loader.java:1612)
    at org.hibernate.loader.Loader.doQuery(Loader.java:717)
    at org.hibernate.loader.Loader.doQueryAndInitializeNonLazyCollections(Loader.java:270)
    at org.hibernate.loader.Loader.doList(Loader.java:2294)
    at org.hibernate.loader.Loader.listIgnoreQueryCache(Loader.java:2172)
    at org.hibernate.loader.Loader.list(Loader.java:2167)
    at org.hibernate.loader.criteria.CriteriaLoader.list(CriteriaLoader.java:119)
    at org.hibernate.impl.SessionImpl.list(SessionImpl.java:1706)
    at org.hibernate.impl.CriteriaImpl.list(CriteriaImpl.java:347)
    at com.BiddingSystem.server.ServiceImpl.UpdateCategory(ServiceImpl.java:799)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
    at java.lang.reflect.Method.invoke(Method.java:597)
    at net.sf.gilead.gwt.PersistentRemoteService.processCall(PersistentRemoteService.java:174)
    ... 21 more