Hi guys, I am having the following exception "Creating a proxy instance failed (Ambiguous match found)", when i try to delete an object

Here is the mapping xml and the class definition, Actually I am using .net frm 4.0,

<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2">
<class name="Resto.Model.Entities.ProductType, Resto.Model" table="ProductType" lazy="true">
<id name="Id" column="Id" type="Int32" unsaved-value="0">
<generator class="identity"/>
</id>
<property column="Description" type="String" name="Description" length="100"/>
<bag name="Childs" lazy="false" cascade="all" inverse="true">
<key column="ParentId" />
<one-to-many class="Resto.Model.Entities.ProductType, Resto.Model" />
</bag>
<many-to-one name="Parent" class="Resto.Model.Entities.ProductType, Resto.Model" column="ParentId" not-null="false" />
</class>
</hibernate-mapping>

public class ProductType : Identificable
{

public ProductType()
{

}
public ProductType(string description)
{
this.Description = description;
}

public virtual string Description { set; get; }
public virtual ProductType Parent { set; get; }
public virtual IList<ProductType> Childs { set; get; }

public virtual void AddChild(ProductType child)
{
child.Parent = this;
this.Childs.Add(child);
}


}