Exception in thread "main" org.springframework.orm.hibernate3.HibernateSystemException: attempted to assign id from null one-to-one property: KMS

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<!-- Generated Sep 11, 2007 3:40:08 PM by Hibernate Tools 3.2.0.b9 -->
<hibernate-mapping>
    <class name="com.t.KMS" table="KMS">
        <id name="instrument" type="big_decimal" >
         <column name="INSTRUMENT" precision="22" scale="0" not-null="true"/>
            <generator class="com.t.IDGenerator" />
        </id>
        <property name="tableId" type="string">
            <column name="TABLE_ID" length="20" not-null="true" />
        </property>
        <property name="creator" type="big_decimal">
            <column name="CREATOR" precision="22" scale="0" not-null="true" />
        </property>
        <property name="createTime" type="date">
            <column name="CREATE_TIME" length="7" not-null="true" />
        </property>
        <property name="instrgroup" type="string">
            <column name="INSTRGROUP" length="20" />
        </property>
        <property name="actor" type="string">
            <column name="ACTOR" length="30" />
        </property>
       
        <one-to-one name="ESI"  class="com.t.ESI" cascade="all" />
    </class>

<class name="com.t.ESI"
      table="ESI">
      <id name="instrument" column="INSTRUMENT" >
         <generator class="foreign">
            <param name="property">KMS</param>
         </generator>
      </id>
       
        <one-to-one name="KMS" class="com.t.KMS" constrained="true" />
      
            <property name="wkn" type="string">
         <column name="WKN" length="12" />
      </property>
      <property name="updateUser" type="big_decimal">
         <column name="UPDATE_USER" precision="22" scale="0"
            not-null="true" />
      </property>
      <property name="updateTime" type="date">
         <column name="UPDATE_TIME" length="7" not-null="true" />
      
            </property>
   </class>

</hibernate-mapping>

import java.io.Serializable;
import java.math.BigDecimal;
import java.util.Properties;

import org.hibernate.HibernateException;
import org.hibernate.MappingException;
import org.hibernate.Query;
import org.hibernate.Session;
import org.hibernate.dialect.Dialect;
import org.hibernate.engine.SessionImplementor;
import org.hibernate.id.Configurable;
import org.hibernate.id.IdentifierGenerator;
import org.hibernate.type.Type;

public class IDGenerator implements IdentifierGenerator{


   public Serializable generate(SessionImplementor sessionImplementor, Object object)
         throws HibernateException {
      
      Session session=sessionImplementor.getFactory().openSession();
      
      String SQL_QUERY="SELECT MAX(instrument)+1 AS instrument FROM KMS "
         + " WHERE instrument > 100000000 AND instrument < 150000000";
      
      Query query=session.createQuery(SQL_QUERY);
      
      BigDecimal instrumentId=(BigDecimal)query.iterate().next();
      
      return instrumentId;
   }

   


}