XML Message Error Handling

So im using the XML below to add REQ_GROUP to new Tasks, works as expected, but generates an error where there is no Request, i.e. Project Task.  It doesnt seem to affect anything, but how do i capture this and not run the update? Thanks

<perform_batch>
  <hierarchy_select result_name="select_task01">
    <primary_table>task</primary_table>
    <attrs>
      <attr>task.task_id</attr>
      <attr>request.req_group</attr>
    </attrs>
    <from>
      <table>request</table>
    </from>
    <where>
      <join_constraint>
        <constraint>
          <left_operand>task.request_id</left_operand>
          <operator>eq</operator>
          <right_operand>request.request_id</right_operand>
        </constraint>
      </join_constraint>
      <data_constraint>
        <constraint>
          <left_operand>task.task_id</left_operand>
          <operator>eq</operator>
          <right_operand>@task_id</right_operand>
        </constraint>
      </data_constraint>
    </where>
  </hierarchy_select>
  <update_task>
    <task>
      <task_id xpath_node="//task_hierarchy_select_result[@result_name='select_task01']/task/task_id" />
      <user_def6 xpath_node="//task_hierarchy_select_result[@result_name='select_task01']/task/request/req_group" />
      <update />
    </task>
  </update_task>
</perform_batch>