At learning Stage I have a problem in following code;

SqlDataSource2.FilterExpression = "fld1 = " + TextBox1.Text; is giving column not error

public partial class OffPage : System.Web.UI.Page
{
    SqlConnection con3 = new SqlConnection(ConfigurationManager.ConnectionStrings["LoginCS"].ConnectionString);
    SqlCommand cmd3 = new SqlCommand();

    protected void Page_Load(object sender, EventArgs e)
    {
       con3.Open();
    }
    protected void Button1_Click(object sender, EventArgs e)
    {
        if (Convert.ToInt32(RadioButtonList1.SelectedValue) == 0)
        {
            TextBox1.Text = "Pending";
            SqlDataSource2.FilterExpression = "fld1 = " + TextBox1.Text;
        }
        if (Convert.ToInt32(RadioButtonList1.SelectedValue) == 1)
        {
            TextBox1.Text = "FWD";
            SqlDataSource2.FilterExpression = "fld1 = " + TextBox1.Text;
        }
        
    }
 }