New Post: When I convert image with thereshold 80% I get a blank page

I'm using version 6.8.8.701 (Magick.NET-x86.dll from zip file)

If I use convert from command line convert -thereshold 80% 1.jpg 2.jpg goes right.

I'm trying the same using library but seems I'm missing something.

using (MagickImage image = new MagickImage(@"C:\test.jpg"))

        {
            image.Threshold(80);
            image.Write(@"C:\test6.pdf");
        }

I've tested also with
using (MagickImage image = new MagickImage(@"C:\test.jpg"))

        {
            image.Threshold(0.8);
            image.Write(@"C:\test6.pdf");
        }

I've been looking at documentation but I've not found the key or my mistake.

Thanks in advance.