Note that this occurred on 3.0.4249 beta.
I had the following line:
deproject_screen_space(x, y, world_location, world_direction, controller, Vector2!xy(viewport_x, viewport_y))
And was getting the error:
(Side-note, it would be great to be able to right-click and copy error messages)
The error message was misleading because
x already was a
Real. It was actually complaining about
viewport_x which I had initialized incorrectly but the message had stripped off
viewport_ which led to my confusion.