Adding text to an image with Elixir Mogrify

I'm having trouble with the Mogrify package in Elixir. I have an image that I want to add text to. For some reason the image will open and save a copy without any issues but I can't get the copy to be changed in any way.

    img_url = to_string(:code.priv_dir(:jobs)) <> "/tristar.png"
    save_url = to_string(:code.priv_dir(:jobs)) <> "/tristar_copy.png"

    Mogrify.open(img_url)
    |> Mogrify.custom("pointsize", 200)
    |> Mogrify.custom("gravity", "North")
    |> Mogrify.custom("annotate", "+0,+100 'Testing'")
    |> IO.inspect(label: "\n===== Image Pre-Save =============================================")
    |> Mogrify.save(path: save_url)

The output in Iex looks like:

===== Image Pre-Save =============================================:
%Mogrify.Image{
  animated: false,
  buffer: nil,
  dirty: %{},
  ext: ".png",
  format: nil,
  frame_count: 1,
  height: nil,
  operations: [
    {"pointsize", 200},
    {"gravity", "North"},
    {"annotate", "+0,+100 'Testing'"}
  ],
  path: "/code/elixir/_build/dev/lib/jobs/priv/tristar.png",
  width: nil
}
%Mogrify.Image{
  animated: false,
  buffer: nil,
  dirty: %{},
  ext: ".png",
  format: nil,
  frame_count: 1,
  height: nil,
  operations: [],
  path: "/code/elixir/_build/dev/lib/jobs/priv/tristar_copy.png",
  width: nil
}

I've tried adding fill and stroke with custom. I've tried using label, draw text, and annotate. The saved image at the end is simply a copy of the original.

You are one comma away from a working solution:

Change "+0,+100 'Testing'" to "+0+100 'Testing'".