为什么收到此错误:指定的强制转换无效

我正在使用此代码在VS2010中打开晶体报告，axCRViewer1 crystal report viewer control name，但是在这一行出现错误

axCRViewer1.ReportSource = rptDoc;

我该如何解决？

private void ViewR_Load(object sender, EventArgs e)
            {
                ReportDocument rptDoc = new ReportDocument();
                DataSetPatient ds = new DataSetPatient(); // .xsd file name
                DataTable dt1 = new DataTable();
                DataTable dt = DBHandling.GetPatient();//getting data using GetPatient()
                // Just set the name of data table
                dt.TableName = "Crystal Report P";
                ds.Tables[0].Merge(dt);
                // Your .rpt file path will be below
                rptDoc.Load("C:''Users''Monika''Documents''Visual Studio 2010''Projects''SonoRepo''SonoRepo''Reports''CrystalReportP.rpt");
                //set dataset to the report viewer.
                rptDoc.SetDataSource(ds);
                axCRViewer1.ReportSource = rptDoc;//getting error at this line 
                // code to get data from the DB           
            }

获取患者（） 代码

public static DataTable GetPatient()
        {
            DataTable patientTable = new DataTable();
            using (OleDbConnection con = new OleDbConnection(@"Provider=Microsoft.Jet.OLEDB.4.0;Data Source=sonorepo.mdb"))
            {
                using (OleDbDataAdapter da = new OleDbDataAdapter(@"SELECT PatientID,PFirstName FROM Patient_Registration", con))
                    da.Fill(patientTable);
            }
            return patientTable;
        }