undefined method `downcase' for nil:NilClass (NoMethodError)

[hazybluedot]

when querying a particular element that does not exist

$ find content/ -type f -name '*.md' | xargs fore list tags                                                                                                              
/usr/local/rvm/gems/ruby-2.0.0-p247/gems/forematter-0.1.0/lib/forematter/commands/list.rb:13:in `each': undefined method `downcase' for nil:NilClass (NoMethodError)
        from /usr/local/rvm/gems/ruby-2.0.0-p247/gems/forematter-0.1.0/lib/forematter/commands/list.rb:13:in `sort_by'
        from /usr/local/rvm/gems/ruby-2.0.0-p247/gems/forematter-0.1.0/lib/forematter/commands/list.rb:13:in `run'
        from /usr/local/rvm/gems/ruby-2.0.0-p247/gems/forematter-0.1.0/lib/forematter/command_runner.rb:6:in `call'
        from /usr/local/rvm/gems/ruby-2.0.0-p247/gems/cri-2.4.1/lib/cri/command_dsl.rb:185:in `block in runner'
        from /usr/local/rvm/gems/ruby-2.0.0-p247/gems/cri-2.4.1/lib/cri/command.rb:298:in `call'
        from /usr/local/rvm/gems/ruby-2.0.0-p247/gems/cri-2.4.1/lib/cri/command.rb:298:in `run_this'
        from /usr/local/rvm/gems/ruby-2.0.0-p247/gems/cri-2.4.1/lib/cri/command.rb:251:in `run'
        from /usr/local/rvm/gems/ruby-2.0.0-p247/gems/cri-2.4.1/lib/cri/command.rb:264:in `run'
        from /usr/local/rvm/gems/ruby-2.0.0-p247/gems/forematter-0.1.0/lib/forematter.rb:36:in `run'
        from /usr/local/rvm/gems/ruby-2.0.0-p247/gems/forematter-0.1.0/bin/fore:10:in `<top (required)>'

[bobthecow]

This doesn't fail for me:

$ find content/ -type f -name "*.md" | xargs fore list undefined

Do you mind digging a bit to see what it is about those items that's failing? Maybe you have a tags element that's null or something?

[hazybluedot]

Indeed, it occurred on a file in which I had defined a 'tags' element with no value:

---
title: Some Title
published: false
kind: article
created_at: '2013-07-30 10:42:15 -0400'
tags:
---