How to pass an integer parameter from main report to subreport?

I don't understand why this isn't working, but I have a main report with parameters:

StartDate=datetime,

EndDate=datetime,

Program=text,

ChartType=text.

In this RDL, I have a chart with Action configured on this bar chart series.  The Action expression is:

="javascript:void(window.open('http://evolvssrs/ReportServer/Pages/ReportViewer.aspx?%2fIncoming%2fCensus+Report+Modifier&rs:Command=Render&rc:Parameters=true&StartDate=" & Parameters!StartDate.Value & "&EndDate=" & Parameters!EndDate.Value & "&Program=" & Fields!program_info.Value & "&ChartType=" & Fields!ChartType.Value & "','_blank'))"

(program_info is a FIELD uniqueidentifier value matching the value of the Program parameter).

So for this subreport, I have tried ChartType parameter as both integer and text, but each time i click on a bar from the main report, it opens a new window with ChartType field empty.  But the other 3 parameters: Start Date, End Date, and Program are all populated.  Why is it doing this and how can I fix please?

Ryan D