Jekyll sort on `sort` field issue

_config.yml

markdown: kramdown
highlighter: rouge

collections:
  tut:
    output: true

_include tutorials.html

{% assign sorted_pages = (site.tut | sort: 'sort') %}

_tut 1.md

title	layout	sort
Title	tutorial	1

This is a test

_tut 2.md

title	layout	sort
Title	tutorial	2

This is a test

stdout

Configuration file: _config.yml
            Source: /Users/nicolasrinaudo/dev/nrinaudo/test/jekyll
       Destination: /Users/nicolasrinaudo/dev/nrinaudo/test/jekyll/_site
 Incremental build: disabled. Enable with --incremental
      Generating... 
  Liquid Exception: stack level too deep in _layouts/tutorial.html
/Library/Ruby/Gems/2.0.0/gems/jekyll-3.1.6/lib/jekyll/renderer.rb:112: stack level too deep (SystemStackError)