运行时出现的不理解问题

各位大虾：
    我在用编程时，程序完成后，运行，点击按钮，运行无故退出。也不显示警告或错误。
代码如下：
Private Sub Button8_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button8.Click
        Dim c As New Color
        Dim cc As New Color
        Dim box1 As Bitmap
        Dim box2 As Bitmap
        box1 = New Bitmap(PictureBox1.Image)
        box2 = New Bitmap(PictureBox1.Image)
        Dim rr, gg, bb, r, g, b, rx, gx, bx, k1, k2 As Integer
        Dim i, j As Integer
        For i = 0 To PictureBox1.Image.Width - 1
            For j = 0 To PictureBox1.Image.Height - 1
                rx = 0
                gx = 0
                bx = 0
                For k1 = -1 To 1
                    For k2 = -1 To 1
                        c = box1.GetPixel(i + k1, j + k2)
                        r = c.R
                        g = c.G
                        b = c.B
                        rx = rx + 1
                        gx = gx + 1
                        bx = bx + 1
                    Next
                Next
                rr = Int(rx / 9)
                gg = Int(gx / 9)
                bb = Int(bx / 9)
                If rr < 0 Then rr = 0
                If rr > 255 Then rr = 255
                If gg < 0 Then gg = 0
                If gg > 255 Then gg = 255
                If bb < 0 Then bb = 0
                If bb > 255 Then bb = 255
                Dim c1 As New Color
                c1 = Color.FromArgb(rr, gg, bb)
                box2.SetPixel(i, j, c1)
            Next
            PictureBox1.Refresh()
            PictureBox1.Image = box2
        Next
    End Sub
请哪位大虾指教，万分感谢。