It should be this:

```
{% defer (url=apiParams('user.search', devSlug=devSlug), id='dev-search') %}
```

Because you left out the `url=`, nunjucks tries to parse the defer block in this way:
```
{% - new block
defer - switch to the builder
( - Begin parsing signature
apiParams - new key with name "apiParams"
( - Invalid token for signature, pop
....Lots of errors get ignored...
%} - end of opening defer block
```
In the builder (`builder.js`), the contents of that bit after `defer` should have been stored as an object named `signature` associated with the page lifecycle. Since the parser didn't return anything, though, that value is `undefined`, which means [this](https://github.com/mozilla/commonplace/blob/master/src/media/js/builder.js#L104) will fail.