Unable to cast object of type 'System.DBNull' to type 'System.Byte[]'.

<%@ WebHandler Language="C#" Class="ImageHandler" %>

using System;
using System.Web;
using System.Configuration;
using MySql.Data.MySqlClient;
public class ImageHandler : IHttpHandler 
{


    string strcon = ConfigurationManager.ConnectionStrings["WelfareDB"].ToString();
    public void ProcessRequest (HttpContext context)
    {
        if (context.Request.QueryString["ImID"] !=null)
        {
            string imgid = context.Request.QueryString["ImID"];
            MySqlConnection con = new MySqlConnection(strcon);
            con.Open();
            string qry = "select image from inchargedetail where inchargeid=@detailid";
            MySqlParameter detailidP = new MySqlParameter("@detailid", imgid);
            MySqlDataReader dr = MySqlHelper.ExecuteReader(con, qry, detailidP);
            dr.Read();
            context.Response.BinaryWrite((Byte[])dr[0]);
            con.Close();
            context.Response.End();
        }
      
    }

    public bool IsReusable
    {
        get
        {
            return false;
        }
    }

}