Rational team concert - I want to show subscribers list in the query output

I wrote a calculated value script to read the value of subscribers and autopopulate a custom attribute (of type string).
The aim was to include the custom attribute in the query output.
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dojo.provide("org.example.workitems.providers.CalculatedSubscribers");
dojo.require("com.ibm.team.workitem.api.common.WorkItemAttributes");
(function() {
var WorkItemAttributes= com.ibm.team.workitem.api.common.WorkItemAttributes;
dojo.declare("org.example.workitems.providers.CalculatedSubscribers", null, {
    getValue: function(attributeId, workItem, configuration) {
       return workItem.getValue(WorkItemAttributes.SUBSCRIPTIONS)
    }
});
})();
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This script does not work. The custom attribute comes blank
-- Is the script correct? Is it possible to "getValues" of "subscriptions" like this?
-- Am I missing upon some steps here? I already included the "subscribedby" attribute as a "dependency" in the custom attribute.