validate the date from the database in asp.net using c#

Database as follows
 
studentid       crbillno                DOM

133                  124290           20-01-2014

In run mode screen as follows
 
Batch Date 28-06-2014
 
i have one button called Submit.
 
When i click the submit i want to validate dropdownlist date must be less than DOB date(DOM).
 
for that my code as follows

DateTime coursedate;
        coursedate = Convert.ToDateTime(DdlBthDates.SelectedItem.Text.ToString());
        Sql = "select DOM from course where stud_id = '" + Session["stud_id"] + "' and CR_BILL_NO  =  '" + Session["cr_bill_no"] + "'";
        coursedr = drr.ExecRdr(Sql);
        if (coursedate < Convert.ToDateTime(coursedr[0].ToString().Trim()))
        {
            Label23.Text = "date must be ";
            return;
        }
        coursedr.Close();

when i run the code shows error as follows
 
Invalid attempt to read no data present.
 
the error shows in below line as follows

 if (coursedate < Convert.ToDateTime(coursedr[0].ToString().Trim()))