Paperclip upload image from localStorage via AJAX, handler error

Question

I store images in localStorage via JS. After a user performs an action, I put the localStorage in a json string and upload it to Amazon S3 via AJAX Post:

var json = {"labelupload":{"label":localStorage['<%= li['_id'] %>'],"label_file_name":'<%= li['_id'] %>.jpg', "li_id":'<%= li['_id'] %>'}}

$.ajax({
  type:'POST',
  url:'/label-upload',
  data: json
})

This works excellent. The image is processed and goes into the controller. However, I always receive a 500 message in my log:

Unpermitted parameter: li_id
Completed 500 Internal Server Error in 1005ms (ActiveRecord: 1.2ms)

Paperclip::AdapterRegistry::NoHandlerError (No handler found for "undefined"):
  app/controllers/carts_controller.rb:109:in `label_upload'

"undefined" seems to be the "label" parameter which is my localStorage:

Parameters: {"labelupload"=>{"label"=>"undefined", "label_file_name"=>"558194ab64356300172a0000.jpg", "li_id"=>"558194ab64356300172a0000"}}

I don't know what to do here. Why is the localStorage considered "undefined" here? How can I get rid off this error message?