Ambiguous error "is this input UTF-8 encoded?"

I am getting the following error on my project file:
```
shared:ERROR: ERROR - [JSC_READ_ERROR]
Cannot read file C:/Users/jukkaj/AppData/Local/Temp/emscripten_temp_my5mvvsw/a.wasm.o.js:
Failed to read: C:\Users\jukkaj\AppData\Local\Temp\emscripten_temp_my5mvvsw\a.wasm.o.js,
is this input UTF-8 encoded?
```
Does this error mean "the input file should be UTF-8 encoded, otherwise Closure cannot read it", or "UTF-8 files will fail to read, the file should not be UTF-8"?