Ошибка Cannot convert from 'ref decimal' to 'ref decimal?

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;
using System.Data.SqlClient;
 
namespace Turfirma
{
    public partial class Form7 : Form
    {
        public Form7()
        {
            InitializeComponent();
        }
 
        private void Form7_Load(object sender, EventArgs e)
        {
            // TODO: This line of code loads data into the 'turfirmaDataSet.AllTuriPoCene' table. You can move, or remove it, as needed.
            this.allTuriPoCeneTableAdapter.Fill(this.turfirmaDataSet.AllTuriPoCene);
        }
 
        private void button1_Click(object sender, EventArgs e)
        {
            try
            {
                decimal Cena = decimal.Parse(this.textBox1.Text);
 
                Form1.myConnection = new SqlConnection();
                Form1.myConnection.ConnectionString = "Data Source=(local);Initial Catalog=Turfirma;Integrated Security=true;User ID=" + Form1.login + ";Password=" + Form1.pass + ";";
 
                Form1.myConnection.Open();
                SqlCommand myCommand = Form1.myConnection.CreateCommand();
                myCommand.CommandType = CommandType.StoredProcedure;
                myCommand.CommandText = "[TuriPoCene]";
                myCommand.Parameters.Add("@Cena", SqlDbType.Decimal, 12);
                myCommand.Parameters["@Cena"].Value = Cena;
                myCommand.ExecuteNonQuery();
               this.allTuriPoCeneTableAdapter.FillBy(this.turfirmaDataSet.AllTuriPoCene, ref Cena);
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.ToString());
            }
            finally
            {
                Form1.myConnection.Close();
            }
        }
    }
}

decimal? Cena = decimal.Parse(this.textBox1.Text);