ExecuteScalar()返回错误“；指定的强制转换无效"；当没有记录时

我使用下面的代码来计算每个客户的(发票总额(，对于没有发票的客户，它返回错误，我试图用null处理错误，但它不起作用。

   public static decimal GetInvoiceTotal(int customerID)
    {
        SqlConnection connection = MMABooksDB.GetConnection();
        string selectStatement
            = "SELECT SUM(InvoiceTotal) "
            + "FROM Invoices "
            + "WHERE CustomerID = @CustomerID";
        SqlCommand selectCommand =
            new SqlCommand(selectStatement, connection);
        selectCommand.Parameters.AddWithValue("@CustomerID", customerID);


        try
        {
            connection.Open();
            if (selectCommand.ExecuteScalar()!=null)
            {
                decimal invoiceTotal = (decimal)selectCommand.ExecuteScalar();
                return invoiceTotal;
            }

            else
            {
                return 0;
            }
        }
        catch (SqlException ex)
        {
            throw ex;
        }
        finally
        {
            connection.Close();
        }
    }

var value = selectCommand.ExecuteScalar();
if(value != DBNull.Value)
{
    return (decimal)value;
}