"An error in aggregation operator kmeans"

Hi,

This is my code:

<operator name="Root" class="Process" expanded="yes">
    <operator name="ExampleSetGenerator" class="ExampleSetGenerator">
        <parameter key="number_examples" value="25"/>
        <parameter key="target_function" value="random"/>
    </operator>
    <operator name="AttributeFilter" class="AttributeFilter">
        <parameter key="condition_class" value="attribute_name_filter"/>
        <parameter key="parameter_string" value="att.*"/>
    </operator>
    <operator name="KMeans" class="KMeans">
        <parameter key="k" value="3"/>
    </operator>
    <operator name="Aggregation" class="Aggregation">
        <list key="aggregation_attributes">
          <parameter key="att1" value="average"/>
        </list>
        <parameter key="group_by_attributes" value="cluster"/>
    </operator>
</operator>

I get the error that the attribute 'cluster' does not exist. But after running the kmeans, a new attribute 'cluster' was created in the exampleset. So, why is this error? Or is it reading the initial input example set ? How do i tell RM to read that particular data which was generated by applying the kmeans?

Thanks, Shubha