ASP.NET(C#) catching exception while using stored procedure (sql database)

The code is as follows... I tried using debug points and it shows:

Procedure or function 'usp_select_legal1_data' expects parameter '@tower', which was not supplied.

C# code:

try {
    LegalView.ActiveViewIndex = 2;

    String tw2 = TextBox3.Text;
    SqlDataSource SqlDataSource2 = new SqlDataSource();
    SqlDataSource2.ID = "SqlDataSource2";
    this.Page.Controls.Add(SqlDataSource2);
    SqlDataSource2.ConnectionString = System.Configuration.ConfigurationManager
        .ConnectionStrings["constr"].ConnectionString;
    SqlDataSource2.SelectParameters.Add("@tower", tw2);
    SqlDataSource2.SelectCommand = "usp_select_legal1_data";

    GVCaseTowerWise.DataSource = SqlDataSource2;
    GVCaseTowerWise.DataBind();

    if (GVCaseTowerWise.Rows.Count == 0) {
        ScriptManager.RegisterClientScriptBlock(
            this,
            this.GetType(),
            "alertMessage",
            "alert('No cases for this tower exist in the database')",
            true);
    }
} catch (Exception ex) {
    ScriptManager.RegisterClientScriptBlock(this, this.GetType(),
            "alertMessage", "alert('error while getting data')", true);
}

And this is my stored procedure:

ALTER PROCEDURE [dbo].[usp_select_legal1_data]
    @tower nvarchar(50)
AS
BEGIN
    SET NOCOUNT ON;

    SELECT distinct
      [tower_to]
      ,[tower_from]
      ,[sy_no]
    FROM [dbo].[legal1]
    WHERE ((tower_to = @tower) or (tower_from = @tower))
END