IMGUI doesn't work on HoloLens?!

Say to a MonoBehaviour of a script attached to an object in my scene I add the simplest bit of IMGUI:

Code (CSharp):

1. public void OnGUI()
2. {
3.     GUILayout.Button("Test");
4. }

In the Unity editor, I see a button. On HoloLens, I see the rest of the scene but no button! How do I get this working?
My goal is to have a GUI.ModalWindow with some labels and buttons working on the HoloLens. That doesn't work on HoloLens either, just like the button, while it works in the Unity editor...