Iam trying to search record using linq but following error are occure

protected void btnsearch_Click(object sender, EventArgs e)
  {
   //   txtid.Enabled = true;

      var chk = context.Students.FirstOrDefault(x=> x.StudentId == Convert.ToInt32(txtid.Text));

      if (chk != null)
      {
          txtname.Text = chk.Name;
          txtrollno.Text = chk.Rollno.ToString();
          ddlcity.SelectedValue = chk.City;
          ddlqualification.SelectedValue = chk.Qualification;
          RBGender.SelectedValue = chk.Gender;
      }
      else
      {
          Response.Write("<script>alert('No such recourd found')</script>");
      }