using SystemParametersInfo to change wallpaper in c#

[stan]

i am using c# (2003) on win2k and xp as administrator. the
following code successfully changes the wallpaper. i am
wondering why i have to set the wallpaper to "(None)"
first? if i do not, it will not change. if i do, it will
change. in either case, SystemParametersInfo returns no
error. in my experience, when i have to include a
seemingly unnecessary extra line of code to do something,
it usually means that i did something wrong earlier. can
anyone spot my error?

using System;
using System.Windows.Forms;
using System.Runtime.InteropServices;

namespace Post
{
public class Form1 : System.Windows.Forms.Form
{
private System.Windows.Forms.Button
button1;

private const int SPI_SETDESKWALLPAPER =
20;
private const int SPIF_UPDATEINIFILE = 0x1;
private const int SPIF_SENDWININICHANGE =
0x2;

[DllImport("user32.dll",
CharSet=CharSet.Auto, SetLastError=true)]
public static extern int
SystemParametersInfo(int uAction , int uParam , string
lpvParam , int fuWinIni);

public Form1()
{
InitializeComponent();
}

#region Windows Form Designer generated
code ... (omitted)
[STAThread]
static void Main()
{
Application.Run(new Form1());
}

private void button1_Click(object sender,
System.EventArgs e)
{
// Why do I have to set it to none
first to make it work???
SystemParametersInfo
(SPI_SETDESKWALLPAPER, 0, "(None)", SPIF_UPDATEINIFILE |
SPIF_SENDWININICHANGE );
SystemParametersInfo
(SPI_SETDESKWALLPAPER, 0, @"C:\WINNT\Soap Bubbles.bmp",
SPIF_UPDATEINIFILE | SPIF_SENDWININICHANGE );
}
}
}