Why Won't My C# Application/Project Retrieve Data From My MS SQL Server 2008 Database And Display it In A Text Box, Help Please.

I Am Writing A Booking System And I want to Retrieve Information Such As Costumer ID and Customer  Name On The Subjects Postcode Store On The Database.
I Have the Following Code
using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using System.Data.SqlClient;
using System.Data.Sql;
using System.Data.SqlTypes;
using System.Data.Odbc;

namespace Plaza_Theater_Booking_System
{

public partial class FrmQueryControl : Form
    {
        //Search Varibles

        //Custumer Names
        string CustumerName1;
        string CustumerName2;
        string CustumerNameF;

        //Custumer Postcode
        string CustumerPostcode;
//Create InputBox
public static DialogResult InputBox(string title, string promptText, ref string value)
        {
            Form form = new Form();
            Label label = new Label();
            TextBox textBox = new TextBox();
            Button buttonOk = new Button();
            Button buttonCancel = new Button();

            form.Text = title;
            label.Text = promptText;
            textBox.Text = value;

            buttonOk.Text = "OK";
            buttonCancel.Text = "Cancel";
            buttonOk.DialogResult = DialogResult.OK;
            buttonCancel.DialogResult = DialogResult.Cancel;

            label.SetBounds(9, 20, 372, 13);
            textBox.SetBounds(12, 36, 372, 20);
            buttonOk.SetBounds(228, 72, 75, 23);
            buttonCancel.SetBounds(309, 72, 75, 23);

            label.AutoSize = true;
            textBox.Anchor = textBox.Anchor | AnchorStyles.Right;
            buttonOk.Anchor = AnchorStyles.Bottom | AnchorStyles.Right;
            buttonCancel.Anchor = AnchorStyles.Bottom | AnchorStyles.Right;

            form.ClientSize = new Size(396, 107);
            form.Controls.AddRange(new Control[] { label, textBox,buttonOk,buttonCancel });
            form.ClientSize = new Size(Math.Max(300, label.Right + 10),    form.ClientSize.Height);
            form.FormBorderStyle = FormBorderStyle.FixedDialog;
            form.StartPosition = FormStartPosition.CenterScreen;
            form.MinimizeBox = false;
            form.MaximizeBox = false;
            form.AcceptButton = buttonOk;
            form.CancelButton = buttonCancel;

            DialogResult dialogResult = form.ShowDialog();
            value = textBox.Text;
            return dialogResult;
        }
//Run Code For Querry Postcode On Click
private void BTNCustumerPostcode_Click(object sender, EventArgs e)
        {
            RTBResults.Text = "";
            string value = "";
            if (FrmQueryControl.InputBox("SQL Varible", "Custumers                   Postcode", ref value) == DialogResult.OK)
            {
                CustumerPostcode = value;
               
            }
            try
            {
                SqlConnection connection = new SqlConnection(@"server =                 TOMDESKPC\SQLEXPRESS; integrated security  = true;                       AsynchronousProcessing=true; database = RAODS Custumer                    SQLData;");
                connection.Open();
                SqlParameter param = new SqlParameter();
                param.ParameterName = "@PostCode";
                param.SqlDbType = SqlDbType.Text;
                param.Value = CustumerPostcode;
                string sql = (@"
                SELECT [Custumer ID], [Title], [First Name], [Second Name],                         [Post Code]
                FROM [RAODS Custumer SQLData].[dbo].[Custumer Data]
                WHERE [Post Code] = '@PostCode'
                ");
                SqlCommand Command = new SqlCommand(sql, connection);
                SqlDataReader rdr = Command.ExecuteReader();
                Command.Parameters.Add(CustumerPostcode);
                while (rdr.Read())
                {
                    RTBResults.Text = "" + rdr[0].ToString();
                    RTBResults.Text += "\t" + rdr[1].ToString();
                    RTBResults.Text += "\t" + rdr[2].ToString();
                    RTBResults.Text += "\t" + rdr[3].ToString();
                    RTBResults.Text += "\t\r\r" + rdr[4].ToString();
                }
                rdr.Close();
                connection.Close();
            }
            catch (SqlException ex)
            {
                MessageBox.Show("Error: Unable To Connect To SQL Server,                 Please Contact Your System Adminstrator. Error : {0} Ocared                    " + ex, "SQL Console", MessageBoxButtons.OK,                             MessageBoxIcon.Error);
            }  
        }

When This Run's I Get The Error Message:

System.InvalidCastException
 
The SqlParameterCollection only accepts non-null SqlParameter type objects, not String objects.

The Data In The SQL Table Is Like This (#### ###) eg (SO51 7HS).

The Varible type for the Database Field [Post Code] is NVarChar.

Any Help Would Be Much Aprceated as i have been trying to solve this all week.

Thank You

Thomas Smythe-Wright