Ionic native: DirectoryEntry.getFile: Expected String, but got Array

At its simplest level, I have this code. (In an ionic 3 app, running ionic native 4.2.2, emulating using android studio)

return this.file.readAsArrayBuffer(url, fileName)
      .then((file) => {
          console.log("HEY. WE FOUND THE FILE")
          return file
      .catch((err) => {

        console.log("ERROR IN genFile")

        return new Blob([res])

File is the import from ‘@ionic-native/file’;

Now, whenever this code executes, even when I manually pass in a url i know to be valid (confirmed its existence with checkFile), the code throws an error, namely: “Wrong type for parameter “path” of DirectoryEntry.getFile: Expected String, but got Array.” According to the docs, both parameters should be strings, and indeed, they are, (this happens when i hardcode the parameters to be strings). I dove into the cordova code, and indeed, there’s a function in DirectoryEntry called getFile.

Now, I believe I’m doing everything alright and this might be a bug, but searching the internet has revealed exactly no-one with this problem, so I thought i’d ask and see if anyone has encountered this before, and how they’ve tried to resolve it.