GRIDVIEW SORTING

Hi friends iam implementing the gridview sorting in my application but sorting is not done for all rows.
it shows a error like "can not fine column name empname".

iam using bellow code
public void BindGVPasswordChange()
    {
        try
        {
            DataTable dt = new DataTable();
            Hashtable htnew = new Hashtable();
            htnew.Add("PwdId", 0);
            dt = _sec.getDataTable("Stp_UM_FillGV_Changepasswords", CommandType.StoredProcedure, htnew);
            if (dt.Rows.Count > 0)
            {
                DataView dv = dt.DefaultView;
                if (ViewState["SortDirection"] != null)
                {
                    sortDirection = ViewState["SortDirection"].ToString();
                }
                if (ViewState["SortExpression"] != null)
                {
                    sortExpression = ViewState["SortExpression"].ToString();
                    string sort = string.Concat(sortExpression, " ", sortDirection);
                    dv.Sort = string.Concat(sortExpression, " ", sortDirection);
                }
                GVChangePwd.DataSource = dv;
                GVChangePwd.DataSource = dt;
                GVChangePwd.DataBind();          
            }
            GVChangePwd.Visible = true;
        }
        catch (Exception ex)
        {
        }
    }


protected void GVChangePwd_Sorting(object sender, GridViewSortEventArgs e)
    {
        if (ViewState["SortDirection"] == null || ViewState["SortExpression"].ToString() != e.SortExpression)
        {
            ViewState["SortDirection"] = "ASC";
            GVChangePwd.PageIndex = 0;
        }
        else if (ViewState["SortDirection"].ToString() == "ASC")
        {
            ViewState["SortDirection"] = "DESC";
        }
        else if (ViewState["SortDirection"].ToString() == "DESC")
        {
            ViewState["SortDirection"] = "ASC";
        }
        ViewState["SortExpression"] = e.SortExpression;
        BindGVPasswordChange();

    }


please help me..