In Ruby on Rails, why "render :json => array_data" shows nothing, but "render :text => array_data.to_json" shows the data array?

Question

In Ruby on Rails, why

render :json => array_data

shows nothing (as seen in Firebug, the GET in the console), but

render :text => array_data.to_json

shows the data array?

This is related to http://stackoverflow.com/questions/3367246/in-ruby-on-rails-how-to-render-json