Issue with script based calculated value in RTC 3.0.1.3.

I have created three new attributes of type "Duration" --  "QA Estimate", "Dev Estimate" and "Total Estimate". The "Total Estimate" attribute uses calculated value from the script TimeEstCalc.js and has dependencies on "QA Estimate" and "Dev Estimate" attributes.

Here is the TimeEstCalc.js script:

dojo.provide("com.example.common.TimeEstCalc");

(function() {

    dojo.declare("com.example.common.TimeEstCalc", null, {

        getValue: function(attribute, workItem, configuration) {

                                             return workItem.getValue("devEstimate") + workItem.getValue("qaEstimate");

        }

    });

})();

After I enter values into "QA Estimate" and "Dev Estimate" fields in the newly created work item, and trying to save the work item, I get the error:

The value of attribute 'Total Estimate' is not valid. Enter a valid duration (w = weeks, d = days, h = hours, m = minutes).

So looks like the "Total Estimate" attribute expects values to be entered instead of returning the calculated value based on the script.

I have script execution enabled in teamserver.properties.

Not sure what I am doing wrong.

Anatoliy