Error: Subreport could not be shown

I am a new one in using ReportViewer in VS 2005. I try to use Main and Sub report to show data from my dataset. It doesn't matter if I use main report only, but the problem is when I use subreport intergrated with the main report. When I run it in browser, it shows:

   Error: Subreport could not be shown

The main report is ok.Is there any solution to the problem?

Thank for your help!!!!