I'd love it if you could test with `undefined` as well. Here's why:

```
{{ foo.bar|translate(foo) }}
```
maps to

```js
filter(member(lookup('foo'), 'bar'), 'translate', [foo])
```
If `bar` is not a member of `foo`, you get `undefined` back out. The filter gets called with the base (what's on the left) as the first argument, so it's possible to get `undefined` out of it. Kind of some Jinja2 weirdness that gets carried over to nunjucks, but it's generally useful.