Can't use variable for basic operations in Nanoc

Question

I got a quite annoying problem tonight, and maybe someone here will be able to help me.

I'm building a static blog with nanoc, and currently making some helpers for next/previous articles (I included some tests I made and the return):

# lib/helpers.rb
include Nanoc3::Helpers::Blogging
include Nanoc3::Helpers::LinkTo

# Returns a link to the next article
# If the article in param is the most recent, returns nothing
def next_article article
  articles = sorted_articles # returns an array of article ordered by date
  pos = articles.index(article) # returns the expected integer
  pos.class # returns Fixnum
  pos.nil? # returns false
  pos + 1 # NoMethodError: undefined method `+' for nil:NilClass
  return if pos.zero? # NoMethodError: undefined method `zero?' for nil:NilClass
  link_to(articles[pos+1][:title], articles[pos+1]) # Fails too, obviously
end

I have absolutely no idea of why I can't use the "pos" variable, but can still perform some read on it. If anyone has an insight, I'll take it. Thanks in advance !

(I use ruby-1.9.3p194, with rvm, on OSX Lion, if it may have any relation)

Update : I should have precised that the returned value for pos is the expected value when just read. Strangely enough, setting

pos = articles.index(article).to_s.to_i

seems to work though. I just don't understand how & why it happens.

#layout/post_navigation.haml
.post_navigation
  .next
    =next_article @item