'System.Data.DataRowCollection.this[int]' has some invalid arguments

Hi
               i am facing a wonderful problem , i can't understand why this error

in my .aspx.cs file

OleDbDataAdapter adpTimeBar = new OleDbDataAdapter("select Emp_Name, Access_Date, Access_Time from Access_Reports ", _OleDbConnString); //_OleDbConnString is string varible used for connection string

        DataTable dtTimeBeta = new DataTable();
        adpTimeBar.Fill(dtTimeBeta);
DateTime AccessDate1 = new DateTime(0001, 01, 01, 00, 00, 00);
        DateTime AccessDate2 = new DateTime(0001, 01, 01, 00, 00, 00);

AccessDate2 = Convert.ToDateTime(dtTimeBeta.Rows[0]["Access_Date"]);
for (UInt32 i = 0; i < dtTimeBeta.Rows.Count; i++)
        {
            AccessDate1 = Convert.ToDateTime(dtTimeBeta.Rows[i]["Access_Date"]);// error is on this line    "System.Data.DataRowCollection.this[int]' has some invalid arguments"

Plese help me its urgent. Thanks in advance