I have a problem with this code... I don't have the slightest clue for what is happening...

When I run this code in Visual Studio, I get an error saying:

> 
> System.InvalidCastException: 'Object cannot be cast from DBNull to other types.'
> 
> 
> 

And here's the code:

```
conn.Open();

SqlCommand com = new SqlCommand("update lend set date_back=convert(datetime2, getdate(), 102) where client_name ='" + comboBox1.Text + "'", conn);

SqlDataAdapter da = new SqlDataAdapter("Declare @startdate smalldatetime declare @enddate smalldatetime set @startdate = (select date_lended from dbo.lend where client_name = '" + comboBox1.Text + "') set @enddate = (select date_back from dbo.lend where client_name = '" + comboBox1.Text + "') SELECT DATEDIFF(DAY, @startdate+2, @enddate)as timepassedd", conn);

DataTable dt = new DataTable();
da.Fill(dt);

foreach (DataRow DR in dt.Rows)
{
    int date;
    date = Convert.ToInt32(DR["timepassedd"]);

    if (date > 0)
    {
        com = new SqlCommand("DELETE lend WHERE client_name ='"+comboBox1.Text+"'" +
                             "UPDATE book_list set book_stock = book_stock  1 WHERE book_name ='" + comboBox1.Text + "'",conn);
        com.ExecuteNonQuery();

        MessageBox.Show("You Returned the book " + date + " Days Late!" +
                            "please pay the fee to the front desk");

        UserPanel u = new UserPanel();
        u.Show();

        this.Hide();
    }
    else if (date <= 0)
    {
        com = new SqlCommand("DELETE lend WHERE client_name ='" + comboBox1.Text + "'" +
                             "UPDATE book_list set book_stock = book_stock  1 WHERE book_name ='" + comboBox1.Text + "'", conn);
        com.ExecuteNonQuery();

        MessageBox.Show("You returned the book " + date + " Days Late!" +
                            "please pay the fee to the front desk");

        UserPanel u = new UserPanel();
        u.Show();

        this.Hide();
    }
}

conn.Close();

```

Thanks in advance