RTC - query workitems where internalSubscriptions.itemId = Current Contributor

Hi,
I am using RTC 4.0.1 (BIRT 2.6.2) to build a time tracking report for the workitems for each person.
Therefore, I need to filter the table com.ibm.team.workitem.WorkItem.
If I provide
internalSubscriptions.userId = '{Current Contributor}'
in the field "Enter filter" is does not work. With an hard-coded name ist works: e.g.
internalSubscriptions.userId = 'any hard coded name'

Other fields like:
 modifiedBy.userId = '{Current Contributor}'
do work properly

Any hints or ideas ?
Thanks, Stef