error in comparing (netsed if else logic)

hi, i am getting problem in validation (selecting maxumim registrationno from table),after we have to compare new registrationno can't be greater than old registration no. (attached Screen shot)

I am getting errors like:"there is no row at position 0","db casting cant be done".

my Code:

if (!String.IsNullOrEmpty(pract.OldRegistrationNo))
{
pract.RegistrationNo = pract.OldRegistrationNo;
}
else
{
pract.RegistrationNo = Request.Form["txtRegNo"];
}

pract.RegistrationDate = Request.Form["Text1"];
DataTable dt1 = new DataTable();
pract.OldRegistrationNo = Request.Form["txtoldregnumber"]; //my code start from here
db.CommandText = "select max (cast(RegistrationNo as int)) as highest from tbl_practitioner
db.CommandType = CommandType.Text;
dt1 = db.ReturnDataTable();


if (Convert.ToInt32(dt1.Rows[0]["highest"]) < Convert.ToInt32(Request.Form["txtoldregnumber"]))
{

NewPID = pract.Add();
}
else
{
Response.Write("<script>alert('Not valid RegistratrionNo');</script>");
//Response.End();

}
}