.NET - SetPixel

Hy,
I found that the old PSet method in VB6 has been replaced with the Bitmap.SetPixel class.
So I tryed the example in the documentation, but it doesn't work. It compiles ok, but at run-time it gives an error - Invalid Argument used (exception in system.windows.forms.dll for VB, system.drawing.dll in C#)

Has anyone been able to use SetPixel? How?

Here is the sample from the documentation
I just put a picturebox on a form and edit the Paint event.

   Private Sub PictureBox1_Paint(ByVal sender As Object, ByVal e As System.Windows.Forms.PaintEventArgs) Handles PictureBox1.Paint
        ' Create a Bitmap object from a file.
        Dim myBitmap As New Bitmap("d:\ok.gif")
        ' Draw myBitmap to the screen.
        e.Graphics.DrawImage(myBitmap, 0, 0, myBitmap.Width, _
        myBitmap.Height)
        ' Set each pixel in myBitmap to black.
        Dim Xcount As Integer
        For Xcount = 0 To myBitmap.Width - 1
            Dim Ycount As Integer
            For Ycount = 0 To myBitmap.Height - 1
                myBitmap.SetPixel(Xcount, Ycount, Color.Black)
            Next Ycount
        Next Xcount
        ' Draw myBitmap to the screen again.
        e.Graphics.DrawImage(myBitmap, myBitmap.Width, 0, myBitmap.Width, _
        myBitmap.Height)

    End Sub

Remarque: 1.if I comment out the SetPixel line, it works.
2. Even if I hardcode it SetPixel(1,1,Color.Black) it doesn't work.