Exception in thread "AWT-EventQueue-1" java.lang.ClassCastException: admintool.data.Diagnose$$EnhancerByCGLIB$$48bb0b3e incompatible with java.util.Set

<hibernate-mapping>
<class name="admintool.data.Behandlung" table="behandlung">
<id name="id" column="id" type="java.lang.Integer">
   <generator class="increment"></generator>
</id>
<set name="diagnose_nr">
   <key column="diagnose_nr"/>
   <one-to-many class="admintool.data.Diagnose"/>
</set>
   <many-to-one name="krankenhaus_nr" column="krankenhaus_nr" class="admintool.data.Krankenhaus"/>
</class>
</hibernate-mapping>

<hibernate-mapping>
<class name="admintool.data.Diagnose" table="diagnose">
<id name="id" column="id" type="java.lang.Integer">
   <generator class="increment"></generator>
</id>
   <many-to-one name="krankenhaus_nr" column="krankenhaus_nr" class="admintool.data.Krankenhaus"/>
   <many-to-one name="patient_nr" column="patient_nr" class="admintool.data.Patient"/>
</class>
</hibernate-mapping>

public class Behandlung {
   
   private int id;
   private Krankenhaus krankenhaus_nr;
   private Diagnose diagnose_nr;
   public Behandlung(){
   }
   public Diagnose getDiagnose_nr() {
      return diagnose_nr;
   }
   public void setDiagnose_nr(Diagnose diagnose_nr) {
      this.diagnose_nr = diagnose_nr;
   }
   public int getId() {
      return id;
   }
   public void setId(int id) {
      this.id = id;
   }
   public Krankenhaus getKrankenhaus_nr() {
      return krankenhaus_nr;
   }
   public void setKrankenhaus_nr(Krankenhaus krankenhaus_nr) {
      this.krankenhaus_nr = krankenhaus_nr;
   }
}