Unhandled system.argumentexception.. Parameter is not valid

Private Sub Button2_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button2.Click

      Dim scale_factor As Single = Single.Parse(txtScale.Text)  
       ' Get the source bitmap.

      Dim bm_source As New Bitmap(originalimage)

            ' Make a bitmap for the result.
            Dim bm_dest As New Bitmap( _
                CInt(bm_source.Width * scale_factor), _
                CInt(bm_source.Height * scale_factor))

            ' Make a Graphics object for the result Bitmap.
            Dim gr_dest As Graphics = Graphics.FromImage(bm_dest)

            gr_dest.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.HighQualityBicubic

            'display the result
            picturebox1.Image = New Bitmap(bm_dest)
    end sub