AML for Part Goal

Hi,

I try to insert cost goal with the following AML :

<AML>

  <Item type="Part" action="merge" where="item_number='115150'">

    <item_number>115150</item_number>

    <name>TEST</name>

    <Relationships>

      <Item type="Part Goal" action="add">

        <related_id>

          <Item type="Part Goal" action="add">

            <goal>Cost</goal>

            <actual_value>2018.69</actual_value>

          </Item>

        </related_id>

      </Item>

    </Relationships>

  </Item>

</AML>

And I am getting following fault :

<![CDATA[Cannot insert the value NULL into column 'GOAL', table 'InnovatorSolutionst.innovator.PART_GOAL';....

Why is that ? GOAL field in AML is properly populated ?

And next question, how would I construct AML if I would like to use on Part Goal item merge instead of add ? I have to update actual costs regulary because they do change over time. 

Thanks for your help !

Bostjan