datatable error in asp.net

SqlConnection con = Database.GetConnection();

SqlCommand cmd = new SqlCommand("spArticleCategoryWise", con);
cmd.CommandType = CommandType.StoredProcedure;
cmd.Parameters.AddWithValue("@cat_id", cat_ID);
SqlDataAdapter da = new SqlDataAdapter(cmd);
DataSet ds = new DataSet();
da.Fill(ds);
DataTable dt = (DataTable)ds.Tables["spArticleCategoryWise"];
int totalrows = dt.Rows.Count;
Label1.Text = totalrows.ToString();

Error:Object reference not set to an instance of an object.