Can't access array element in HTMLBars template

As I understand it you should be able to access array elments via {{array.[0].something}}.

I am trying to use this but it is not working. In my case, claims is an async relationship of invoice.

{{invoice.claims.length}} //Returns "1" as expected

All these are blank

{{invoice.claims.[0]}}
{{invoice.claims.[0].claimNumber}}
{{invoice.claims.content.[0]}}
{{invoice.claims.content.[0].claimNumber}}

I can see the claim has been downloaded in ember-inspector but for some reason the binding has not worked.