to_json syntax help

How can I include the delivery_schedule attribute in this?

      format.json { render :json => @campaign.to_json(:include =>
{:category => {:include => :subscriptions}}, :methods => :status) }

I tried this but I get an error:

     format.json { render :json => @campaign.to_json(:include =>
[{:category => {:include
=> :subscriptions}}, :delivery_schedule], :methods => :status) }

NoMethodError (You have a nil object when you didn't expect it!
The error occurred while evaluating nil.macro):
    /vendor/rails/activerecord/lib/active_record/serialization.rb:
55:in `add_includes'

Note: this works (I removed the category.subscriptions include):

format.json { render :json => @campaign.to_json(:include =>
[:category, :delivery_schedule], :methods => :status) }

Is there something wrong with my nesting?