Showing data type mismatch in this code

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Data.OleDb;
using System.Data;
using System.Configuration;
using System.IO;

public partial class addsize : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
        {
            {
                string ConnectionString = "Provider=Microsoft.ACE.OLEDB.12.0;Data Source=" + Server.MapPath("~/Database/registration.accdb");
                using (OleDbConnection con = new OleDbConnection(ConnectionString))
                {
                    OleDbDataAdapter da = new OleDbDataAdapter(" SELECT * FROM tblcategory", con);
                    DataSet ds = new DataSet();
                    da.Fill(ds);
                    DropDownList1.DataSource = ds.Tables[0];
                    DropDownList1.DataTextField = "catname";
                    DropDownList1.DataValueField = "catid";
                    DropDownList1.DataBind();
                    DropDownList1.Items.Insert(0, new ListItem("--Select--", "0"));
                }
            }
        }
    }
    protected void Button1_Click(object sender, EventArgs e)
    {
        {
            OleDbConnection con = new OleDbConnection();
            con.ConnectionString = "Provider=Microsoft.ACE.OLEDB.12.0;Data Source=" + Server.MapPath("~/Database/registration.accdb");
            con.Open();

            OleDbDataAdapter SQLAdapter = new OleDbDataAdapter("insert into size([sizename]) values('" + TextBox1.Text + "' )", con);
            DataTable DT = new DataTable();
            SQLAdapter.Fill(DT);
            Response.Write("Size Added Successfully");
            TextBox1.Text = string.Empty;
            TextBox1.Text = "";
            DropDownList1.ClearSelection();
            DropDownList1.Items.FindByValue("0").Selected = true;


        }
    }
    protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
    {
        int maincategoryid = Convert.ToInt32(DropDownList1.SelectedItem.Value);
        string ConnectionString = "Provider=Microsoft.ACE.OLEDB.12.0;Data Source=" + Server.MapPath("~/Database/registration.accdb");
        using (OleDbConnection con = new OleDbConnection(ConnectionString))
        {
            OleDbDataAdapter ds = new OleDbDataAdapter(" SELECT * FROM subcategory where maincatid='"+DropDownList1.SelectedItem.Value+"'", con);
            DataSet dt = new DataSet();
            ds.Fill(dt);//error is here
            DropDownList2.DataSource = dt.Tables[0];
            DropDownList2.DataTextField = "subcatname";
            DropDownList2.DataValueField = "subcatid";
            DropDownList2.DataBind();
            DropDownList2.Items.Insert(0, new ListItem("--Select--", "0"));
        }
    }
}