OK, I just got this same error.  ruby 1.8.2 on linux (somewhat updated
FC1).  It borks on 'ri Array', Hash, File and probably more.  But
things like 'ri Array#hash' work.  I've installed ruby by hand in
/usr, and started installing libs (and their docs in --ri-site).  For
example, 'ri NArray' works.

I don't think I'm being a dolt (although if I am, please throw me a
bone) cause I've deleted the docs, and recreated them several times. 
Specifically:
  % rm -r /usr/share/ri/1.8/system
  % cd <src>
  % rdoc --all --ri-system

Now the weird part.  'ri Array' works while the 'Generating RI...' is
showing, and then borks with the below error msg after the rdoc is
complete.

/usr/lib/ruby/1.8/rdoc/ri/ri_descriptions.rb:99:in `concat': cannot
convert nil into Array (TypeError)
        from /usr/lib/ruby/1.8/rdoc/ri/ri_descriptions.rb:99:in `merge_in'
        from /usr/lib/ruby/1.8/rdoc/ri/ri_reader.rb:56:in `get_class'
        from /usr/lib/ruby/1.8/rdoc/ri/ri_reader.rb:52:in `each'
        from /usr/lib/ruby/1.8/rdoc/ri/ri_reader.rb:52:in `get_class'
        from /usr/lib/ruby/1.8/rdoc/ri/ri_driver.rb:70:in `report_class_stuff'
        from /usr/lib/ruby/1.8/rdoc/ri/ri_driver.rb:106:in `get_info_for'
        from /usr/lib/ruby/1.8/rdoc/ri/ri_driver.rb:135:in `process_args'
        from /usr/lib/ruby/1.8/rdoc/ri/ri_driver.rb:134:in `each'
        from /usr/lib/ruby/1.8/rdoc/ri/ri_driver.rb:134:in `process_args'
        from /usr/bin/ri:48

And nope, this was a fresh install.  No old libraries around (or other
docs to confuse it).  Any other ideas?  Thanks in advance.

Cameron