String cannot be of zero length. Parameter name: oldValue1

Hi all,    i got some errors while am creating search option in my application , in this am handling asp.net application using c#.The thing is i enter some text in my textbox and i click search button its working fine, but without entering anything in the textbox and click search button ,its showing following error, "String cannot be of zero length. Parameter name: oldV

click event code,

 protected void imgSearch_Click(object sender, ImageClickEventArgs e)
    {
        Session.Add("searchterm", txtSearch.Text.Trim());
        Response.Redirect("cheers_search-result.aspx");

    }

please see my below source code and let me know if any solution

   clsOleCon objOle = new clsOleCon();
    System.Text.StringBuilder strtodisplay = new System.Text.StringBuilder();
    protected void Page_Load(object sender, EventArgs e)
    {
        string searchterm = Session["searchterm"].ToString();
        
        string query = "SELECT * from content where contents like '%" + searchterm + "%';";
        DataTable dt = objOle.selectquery(query);

        if (dt.Rows.Count > 0)
        {
           
            strtodisplay.Append("

"); foreach (DataRow dr in dt.Rows) { string resultant = ""; if (dr.ItemArray[1].ToString().Length > 150) { resultant = dr.ItemArray[1].ToString().Remove(150, (dr.ItemArray[1].ToString().Length - 150)); resultant = resultant.ToLower(); resultant = resultant.Replace(searchterm, "" + searchterm + ""); // resultant = resultant.Replace(searchterm, "" + searchterm + ""); } else { resultant = dr.ItemArray[1].ToString(); resultant = resultant.Replace(searchterm, "" + searchterm + ""); } resultant = resultant + " ...."; strtodisplay.Append(""); strtodisplay.Append(""); strtodisplay.Append(""); } strtodisplay.Append("

");
            lblSearchresult.Text = strtodisplay.ToString();
        }

        else
        {

            strtodisplay.Append("

");

            lblSearchresult.Text = strtodisplay.ToString();

        }
       
     
        
    }

 

thanks in advance