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

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

using System;
using System.Web;
using System.Data.OracleClient;
using System.Data;
using System.IO;
using System.Web.UI.WebControls;

public class Handler : IHttpHandler
{

    public void ProcessRequest(HttpContext context)
    {
        string id = context.Request.QueryString["ImID"];
        if (id == string.Empty)
        {
            return;
        }
        else
        {
            OracleConnection connection = new OracleConnection(System.Web.Configuration.WebConfigurationManager.ConnectionStrings["OracleConnect1"].ConnectionString);
            connection.Open();
            OracleCommand command = new OracleCommand("select CTD_SHAPE_IMG from  V_CAB_TAG_DTLS where CTD_SHAPE_ID='" + id + "'", connection);
            OracleDataReader dr = command.ExecuteReader();
            dr.Read();
            string str = Convert.ToString(dr["CTD_SHAPE_IMG"]);
            if (Convert.ToString(str) != null)
            {
                context.Response.BinaryWrite((Byte[])dr[0]);
                connection.Close();
                context.Response.End();
            }
        }
    }

    public bool IsReusable
    {
        get
        {
            return false;
        }
    }

}