"Invalid switch" error when running studio from the command line

I was delighted to discover I can run Studio from the command line. Will speed up my workflow no end if I can get it to work, as I have to create lots of single-file projects from different locations as part of my institution's workflow. But I get an "Invalid switch" error. I followed the instructions at docs.sdl.com/.../GUID-26FC92FB-38BA-4AC8-9DE2-D017C687E746 (but changing the "Studio4" folder to "Studio5", as per my version). It's finding the SDLTradosStudio.exe file, but it doesn't like the openDocument switch. I've tried with and without quotation marks around the file I want to translate. The /help switch is working, and the window that opens shows the /openDocument switch, but I can't get it to work. I just get "Invalid Switch". Please advise.