Unfortunately when I tried this again today I couldn’t reproduce the error. However, as I wrote that last sentence I realized the difference between the code now and before is that previously I had a stray dune-project file in the bin directory.
If you clone the sample repository I created and copy the dune-project file into the bin directory you can reproduce the error. To your point about not needing to open Lib - you’re right. I tested things out today and it works properly without opening the library.
Thanks!