You need to add the param to the subreport.  I usually do this by setting up a subreport link to the parameter.  If I don't need the param value in the selection criteria for the subreport, I then remove it from the selection criteria.  This leaves the link set up so that the parameter is sent to the subreport from the main report.  If you do this, I don't think you need to actually set the parameter value for the subreport in your code.

 

-Dell