"prettierInstance.getFileInfo is not a function" for unsaved buffers

[Asheq]

[coc.nvim] TypeError: prettierInstance.getFileInfo is not a function

That's the error I get when I try to format a buffer using <Plug>(coc-format).

The buffer is not saved to a file on disk, but I set the filetype (for example set filetype=json). I think this should be enough information to format.

If I save the buffer to a file on disk (for example ~/Desktop/something.json), then formatting works without a problem. But I don't want to have to do this just to format some scratch buffer.