mitmproxy import doesn't work?

I'm using `mitmproxy` version 12.2.1 (macos, installed through brew) and trying to import flows just throws an error:

```
vespasian: error: import failed: mitmproxy importer: expected JSON array or object, got "2"
```

This doesn't seem to change if I use `--verbose` so any help trying to figure it out would be great 😄 