Work Item customization: is it possible to use Work Item Status attribute as a depedency for triggering a Calculated Value Script?

Hello,
I'm creating a script to store the previous work item state in a custom attribute. The goal is to use this attribute to prevent users from changing the state of an interrupted work item to something other than its previous state. My code so far is quite simple:

dojo.provide("com.company.alm.workitem.previousState");

dojo.require("com.ibm.team.workitem.api.common.WorkItemAttributes");

(function() {

   var WorkItemAttributes = com.ibm.team.workitem.api.common.WorkItemAttributes;

   dojo.declare("com.company.alm.workitem.previousState", null, {

      getValue: function(attributeId, workItem, configuration) {

         var prevState = workItem.getLabel(WorkItemAttributes.STATE);

         return prevState;

      }

   });

})();

The issue I'm facing happens when I configure the previous state custom attribute to use the work item Status as the dependency. Nothing happens. I can change the work item state as many times as I want, but the previous state custom attribute won't change. Now if I change the dependecy to some other work item attribute, such as Description or Owner, the script works fine and stores the work item current state.
I tried using the work item Modified attribute, but it didn't work either. Is my script missing anything? Or using the work item Status as a dependency is not supported?
BTW I'm running RTC 5.0.2 iFix019.

Thanks!