How to properly use the mongo update operator?

I wanted to use the mongo update operator for some simple process, but it keeps stating there is something wrong with the JSON format I provide, even in the simplest format possible. My assumption is therefore that I must have misunderstood how it works, but there are no examples available.

This is an extreme simple example that fails, can someone tell me how I need to use it properly?

<?xml version="1.0" encoding="UTF-8"?><process version="8.1.003">
  <context>
    <input/>
    <output/>
    <macros/>
  </context>
  <operator activated="true" class="process" compatibility="8.1.003" expanded="true" name="Process">
    <process expanded="true">
      <operator activated="true" class="text:create_document" compatibility="8.1.000" expanded="true" height="68" name="Create Document (2)" width="90" x="179" y="136">
        <parameter key="text" value="{&quot;key&quot; : &quot;value&quot;}"/>
      </operator>
      <operator activated="true" class="text:create_document" compatibility="8.1.000" expanded="true" height="68" name="Create Document (3)" width="90" x="179" y="34">
        <parameter key="text" value="{&quot;_id&quot; : &quot;dummy&quot;}"/>
      </operator>
      <operator activated="true" class="nosql:mongodb_document_updater" compatibility="7.6.000" expanded="true" height="82" name="Update MongoDB (3)" width="90" x="380" y="34">
        <parameter key="mongodb_instance" value="my_mongo_db"/>
        <parameter key="collection" value="my_collection"/>
        <parameter key="update_individual_fields" value="false"/>
        <parameter key="insert_unmatched_documents_(upsert)" value="true"/>
      </operator>
      <connect from_op="Create Document (2)" from_port="output" to_op="Update MongoDB (3)" to_port="update"/>
      <connect from_op="Create Document (3)" from_port="output" to_op="Update MongoDB (3)" to_port="criteria"/>
      <portSpacing port="source_input 1" spacing="0"/>
      <portSpacing port="sink_result 1" spacing="0"/>
    </process>
  </operator>
</process>

Note that I have no problem using the other operators, in general I use the execute mongoDB command to perform updates but I'm puzzled with what goes wrong with this one.