Cannot generate a non-primary-key field in hibernate

[Renato Bobbio Calogero]

Good day every body . I got stuck with hibernate.
I mapped my bean as follows :

Code:

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
        "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
        "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">

<hibernate-mapping package="previtc.model.task">
    <class name="PcTask" table="PC_TASK" dynamic-update="true" dynamic-insert="true">
        <id name="taskId" column="TASKID" type="string">
            <generator class="sequence">
                <param name="sequence">PC_TASK_ID_SEQ</param>
            </generator>
        </id>
        <property name="taskNumber" generated="insert" type="java.lang.Integer" insert="false" update="false">
            <column name="TASKNUMBER" not-null="true" unique="true" />
        </property>
        <property name="acquiredBy"         column="ACQUIREDBY"                 type="string" />
        <property name="approvers"          column="APPROVERS"                  type="string" />
        [... other fields ...]
    </class>
</hibernate-mapping>

I defined on my Oracle 10G database that my column TASKNUMBER has a default value = 1,
and on my schema I have the field TASKNUMBER as nullable=false. But when I try to insert in my database an object of the type
PCTASK , I got complained that

could not set a field value by reflection setter of previtc.model.task.PcTask.taskNumber

This is obviously because when I instantiate an object PcTask, I do not assign any value to his TASKNUMBER field, because I want
it to be generated at the moment of the Insert statement.

Do you have any ideas on why hibernate is not automatically generate my field when I try to insert my pcTask object ? Thanks a lot!