ASP.Net Crystal Report Error: Object reference not set to an instance of an object

this code gives me error Object reference not set to an instance of an object

using (ReportDocument report = new ReportDocument())
{
    report.Load(Server.MapPath("~/Crystal Reports/CrReport10.rpt"));
    ViewState["dtgrid"] = dt_grid;
    report.SetDataSource(dt_grid);
    crvCMSettleBill.RefreshReport();
    crvCMSettleBill.ReportSource = report;
    crvCMSettleBill.ReuseParameterValuesOnRefresh = true;
}

But report works fine when coding like this without using using statement.

ReportDocument report = new ReportDocument();
report.Load(Server.MapPath("~/Crystal Reports/CrReport10.rpt"));
ViewState["dtgrid"] = dt_grid;
report.SetDataSource(dt_grid);
crvCMSettleBill.RefreshReport();
crvCMSettleBill.ReportSource = report;
crvCMSettleBill.ReuseParameterValuesOnRefresh = true;

please help me sort out this issue..

thanks regards