Operation illegal on linked parameter

I have a VS2003 Crystal 2008 app that reads a rpt file, assigns the parameter values and exports it to pdf. It works fine, except with subreports with linked parameters. The failure is thrown in the export to PDF "Operation illegal on linked parameter"

How do we set the parameter value of subreport? I've read on previous posts in the forum not to set any value at all (check if the parameter is linked and do not assign any value). I wonder then how does Crystal sets the subreport linked parameter value.

Many thanks,

ILopez