spara som bild

Har lite kod:

Dim SaveDlg As SaveFileDialog = New SaveFileDialog()
        Dim bm As Bitmap = Picture1.Image()
        SaveDlg.Filter = "BMP Files|*.bmp"
        SaveDlg.InitialDirectory = Directory.GetCurrentDirectory
        SaveDlg.ShowDialog()
         If SaveDlg.FileName <> "" Then
            bm.Save(SaveDlg.FileName, System.Drawing.Imaging.ImageFormat.Bmp)
        End If

Den ska alltså spara en picturebox (picture1) som en bmp fil. Men jag får

Object reference not set to an instance of an object."

Why is that?