drpdn SelectedValue is invalid because not exist in list

here is my code

protected void ddlDDLQualifi()

{

try

{

//open the db connection if it is closed...

if (con.State == ConnectionState.Closed)

con.Open();

cmd = new SqlCommand();

cmd.CommandText = "sp_Get_Qualification";

cmd.CommandType = CommandType.StoredProcedure;

cmd.Connection = con;

DDLQualifi.DataSource = cmd.ExecuteReader();

DDLQualifi.DataTextField = "Education_Type";

DDLQualifi.DataValueField = "Education_Id";

DDLQualifi.DataBind();

DDLQualifi.Items.Insert(0, new ListItem("--Select--", "0"));

}

catch (Exception ex)

{

throw ex;

}

finally //Close db Connection if it is open....

{

if (con.State == ConnectionState.Open)

con.Close();

}

the value is binded and it shows value when i saw while debugging but it shows error like this"DDLQualifi' has a SelectedValue which is invalid because it does not exist in the list of items. Parameter name: value"