BD Website Update

I'm in need of help because I'm making a website where: I show the values of the DB lines and I have the option of updating the data via the Web.

When I try to upgrade it gives me the following error:

  

Description: Unhandled exception when running the current Web request. See the stack trace for more information about the error and its source point in the code.

     

Exception Details: System.ArgumentException: Can not set a null value for the 'Salary' column. Instead, use DBNull.

The code excerpt is this:

 protected void Grid2_RowUpdating(object sender, GridViewUpdateEventArgs e)
{
    GridViewRow r1 = Grid2.Rows[e.RowIndex];
    rtrab = tabtrab.FindById((int)Grid2.DataKeys[e.RowIndex].Value);
    if (rtrab != null)
    {
        rtrab["Salario"] = ((TextBox)r1.FindControl("TxtSalario"));
    }
    Grid2.EditIndex = -1;
    adpt1 = new tb_trabalhadorTableAdapter();
    adpt1.Update(tabtrab);
    adpt1.Filltrabalhador(tabtrab);
    enchegrid();
}

The error line is this:

   {
        rtrab["Salario"] = ((TextBox)r1.FindControl("TxtSalario"));
    }