Program indicators - attributes not appearing?

Hi,
I’m trying to make a program indicator (“current age”) using a tracked entity attribute (date of birth). I assume(d) this would be as simple as setting the program indicator expression to:

d2:yearsBetween(attribute, current date)

However, the list of attributes tab in the “expression” part of the setup is empty. Looking at the play.dhis2.org/demo, it looks like it only lists number-type attributes?

I tried to set up the expression anyway (following the example on /demo: https://play.dhis2.org/demo/dhis-web-maintenance-program/showUpdateProgramIndicator.action?id=423463), but it does not work - the attribute value is not put into the expression:

Expression with id action:dSBYyCUjCXd was successfully run. Original condition was: d2:daysBetween(A{iESIqZ0R0R0},V{event_date}) - Evaluation ended up as:d2:daysBetween(‘’,‘2016-09-20’) - Result of evaluation was:NaN

Is this a bug, or is it not supported (anymore)?

Olav