symfony1.2で更新処理時に「Cannot insert a value for auto-increment primary key」エラーが出る

Cannot insert a value for auto-increment primary key (fuge.ID)

public function updateNumber($updateNum)
{
    return $this
        ->setNumber($updateNum)
        ->save();
}

public function save(PropelPDO $con = null)
{
~~~略~~~
  $con->beginTransaction();
  try {
    $affectedRows = $this->doSave($con);
    $con->commit();
  foreach (sfMixer::getCallables('BaseFuge:save:post') as $callable)
  {
    call_user_func($callable, $this, $con, $affectedRows);
  }

    FugePeer::addInstanceToPool($this);
    return $affectedRows;
  } catch (PropelException $e) {
    $con->rollBack();
    throw $e;
  }
~~~略~~~
}

protected function doSave(PropelPDO $con)
{
~~~略~~~
  if ($this->isNew() ) {
    $this->modifiedColumns[] = FugePeer::ID;
  }

  // If this object has been modified, then save it to the database.
  if ($this->isModified()) {
    if ($this->isNew()) {
      $pk = FugePeer::doInsert($this, $con);
      $affectedRows += 1; // we are assuming that there is only 1 row per doInsert() which
             // should always be true here (even though technically
             // BasePeer::doInsert() can insert multiple rows).

      $this->setId($pk);  //[IMV] update autoincrement primary key

      $this->setNew(false);
    } else {
      $affectedRows += FugePeer::doUpdate($this, $con);
    }

    $this->resetModified(); // [HL] After being saved an object is no longer 'modified'
  }
~~~略~~~
}

  private $_new = true;

~~~略~~~

  public function isNew()
  {
    return $this->_new;
  }

    /**
     * @return bool
     */
    public function isNew()
    {
        return (strtotime($this->getStartAt() . '+' . sfConfig::get('app_fuge_newAt') . ' day') >= strtotime('now'));
    }