Adding new parameter to existing BIRT Report (Average Cycle Time)

I want to add new parameter to the Average Cycle Time BIRT report. The parameter is custom attribute of Enumeration type. Custom attribute details exist as part of LIVE_WORKITEM_CNT whereas Avg cycle time report uses WORKITEM_AGING_TRENDS to get the WI_COUNT.
I created 2 new data sets to fetch the value of custom attribute and also created cascading parameter group. The problem is the custom attribute values are not appearing in the parameter list when report is run. It is blank.

Following error thrown when report is run
+ An exception occurred during processing. Please see the following message for details:
Failed to prepare the query execution for the data set: Work Items Set
Cannot set the string value () to parameter 13.
    org.eclipse.datatools.connectivity.oda.OdaException: Unexpected string parameter

Please help let me know how to add custom attribute parameter to the birt report. Thanks in advance.